Answers.Body: <p>Not exactly what you're looking for, but in his 1987 match vs Karpov, Kasparov forgot to press his clock during time pressure. Kasparov finally lost that game, the result being heavily influenced by the clock's incident.</p>
  <p><a href="https://www.latimes.com/archives/la-xpm-1987-10-15-mn-14340-story.html" rel="noreferrer">https://www.latimes.com/archives/la-xpm-1987-10-15-mn-14340-story.html</a></p>
  <p><a href="https://apnews.com/d00b24ebff5eede461c2587317bcef62" rel="noreferrer">https://apnews.com/d00b24ebff5eede461c2587317bcef62</a></p>
  <p>That a professional chess player forgets to press his clock after making a move is quite an extraordinary event.</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T20:17:22.013Z
Answers.Text: Paywall. Needs to be a subscriber, and they don't have a free registration. :(
Answers.UserDisplayName: JCCyC
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T21:01:34.860Z
Answers.Text: Hm, that's weird, since I don't have an account or anything like that. Myabe I can access it because I'm from out the US? Ok, try this one; https://www.csmonitor.com/1987/1104/lchs04.html
  If that one doesn't work either, try googling for "karpov kasparov 1987 clock" or something similar.
Answers.UserDisplayName: emdio
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T23:00:31.943Z
Answers.Text: I was able to access both, and I am in the US.
Answers.UserDisplayName: Akavall
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-10T16:35:38.093Z
Answers.Text: Read the AP article. Jeez. Karpov is a dick. Or not. Maybe he DID make that "mistake" of "inadvertently" warning Kasparov on purpose. We'll never know.
Answers.UserDisplayName: JCCyC
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T19:01:31.650Z
Answers.LastActivityDate: 2020-09-07T21:08:34.123Z
Answers.LastEditDate: 2020-09-07T21:08:34.123Z
Answers.UserDisplayName: emdio
Answers.Body: <p>We'll never know the whole truth of the 1951 world championship match. Years after the Soviet Union collapsed, and nearing death, Bronstein <em>still</em> wouldn't talk about it.</p>
  <p>The challenger Bronstein was the son of an imprisoned Jewish dissident, while champion Botvinnik was a model Soviet (dichotomy much the same as Korchnoi and Karpov in the '80s). In the memoir <em>The Sorceror's Apprentice</em>, all Bronstein would say was that he was &quot;under great psychological pressure, and it was up to [him] how [he] dealt with it.&quot;</p>
  <p>We've speculated over 57. Kc2?? in Game 6 for decades, and whether Bronstein resigned prematurely in Game 23 is also a source of doubt.</p>
  <p>I don't think Bronstein felt he was permitted to win that match. And he didn't <em>lose</em> it, he <em>tied</em> it. Like he knew all along he could've taken the title away if he thought he and his family could survive that.</p>
  <pre><code>rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
  [Event &quot;Botvinnik - Bronstein World Championship Match&quot;]
  [Site &quot;Moscow URS&quot;]
  [Date &quot;1951.03.26&quot;]
  [Round &quot;6&quot;]
  [Result &quot;0-1&quot;]
  [White &quot;David Bronstein&quot;]
  [Black &quot;Mikhail Botvinnik&quot;]
  
  1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 d6 6.Bg5 e6 7.Qd2
  h6 8.Bxf6 gxf6 9.O-O-O a6 10.f4 Bd7 11.Kb1 Be7 12.Be2 Nxd4
  13.Qxd4 Qa5 14.Rhf1 h5 15.Rf3 Qc5 16.Qd2 Bc6 17.Re3 Qa5 18.Bf3
  O-O-O 19.Qd3 Rd7 20.h4 Kb8 21.a3 Bd8 22.Ka2 Qc5 23.Re2 a5
  24.a4 Bb6 25.b3 Rc8 26.Qc4 Qxc4 27.bxc4 Rh8 28.Kb3 Rdd8 29.Rd3
  Bg1 30.Red2 Kc7 31.Ne2 Bf2 32.Rd1 Bc5 33.Ng3 Rdg8 34.Ne2 Rh7
  35.f5 e5 36.Nc3 Bd4 37.Rxd4 exd4 38.Rxd4 Rhg7 39.Ne2 Rxg2
  40.Bxg2 Rxg2 41.Nf4 Rg3+ 42.Kb2 Rg4 43.Nxh5 Rxh4 44.Nxf6 Kb6
  45.Rxd6 Kc5 46.e5 Rd4 47.Rxd4 Kxd4 48.Ng4 Bxa4 49.e6 fxe6
  50.f6 Be8 51.Kb3 e5 52.c3 Ke4 53.Nh6 Kf4 54.f7 Bxf7 55.Nxf7 e4
  56.Nd8 e3 57.Kc2?? Kg3 0-1
  </code></pre>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-08T02:17:23.210Z
Answers.LastActivityDate: 2020-09-08T15:59:32.690Z
Answers.LastEditDate: 2020-09-08T15:59:32.690Z
Answers.UserDisplayName: friscodelrosario
Answers.Body: <p>Kasparov's <code>7...h6?</code> in the game below cost him the game and the match, and later Kasparov accused Deep Blue team of cheating.</p>
  <pre><code>[FEN &quot;&quot;]
  [Event &quot;IBM Man-Machine&quot;]
  [Site &quot;New York, NY USA&quot;]
  [Date &quot;1997.05.??&quot;]
  [EventDate &quot;?&quot;]
  [Round &quot;6&quot;]
  [Result &quot;1-0&quot;]
  [White &quot;Deep Blue (Computer)&quot;]
  [Black &quot;Garry Kasparov&quot;]
  [ECO &quot;B17&quot;]
  [WhiteElo &quot;?&quot;]
  [BlackElo &quot;?&quot;]
  [PlyCount &quot;37&quot;]
  
  1.e4 c6 2.d4 d5 3.Nc3 dxe4 4.Nxe4 Nd7 5.Ng5 Ngf6 6.Bd3 e6
  7.N1f3 h6 8.Nxe6 Qe7 9.O-O fxe6 10.Bg6+ Kd8 11.Bf4 b5 12.a4
  Bb7 13.Re1 Nd5 14.Bg3 Kc8 15.axb5 cxb5 16.Qd3 Bc6 17.Bf5 exf5
  18.Rxe7 Bxe7 19.c4 1-0
  </code></pre>
  <p><a href="https://www.chessgames.com/perl/chessgame?gid=1070917" rel="noreferrer">https://www.chessgames.com/perl/chessgame?gid=1070917</a></p>
  <p>I would also like to suggest a move from a final game of the tie-break in Kramnik - Topalov from classical-Fide championship unification match in 2006. With <code>44...Rxc5</code> Topalov went from difficult position to instantly losing. The match was full of controversy if this is what you are looking for.</p>
  <pre><code>[FEN &quot;&quot;]
  [Event &quot;Kramnik - Topalov World Championship Match&quot;]
  [Site &quot;Elista RUS&quot;]
  [Date &quot;2006.10.13&quot;]
  [EventDate &quot;?&quot;]
  [Round &quot;16&quot;]
  [Result &quot;1-0&quot;]
  [White &quot;Vladimir Kramnik&quot;]
  [Black &quot;Veselin Topalov&quot;]
  [ECO &quot;D47&quot;]
  [WhiteElo &quot;?&quot;]
  [BlackElo &quot;?&quot;]
  [PlyCount &quot;89&quot;]
  
  1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 e6 5.e3 Nbd7 6.Bd3 dxc4 7.Bxc4
  b5 8.Be2 Bb7 9.O-O Be7 10.e4 b4 11.e5 bxc3 12.exf6 Bxf6
  13.bxc3 c5 14.dxc5 Nxc5 15.Bb5+ Kf8 16.Qxd8+ Rxd8 17.Ba3 Rc8
  18.Nd4 Be7 19.Rfd1 a6 20.Bf1 Na4 21.Rab1 Be4 22.Rb3 Bxa3
  23.Rxa3 Nc5 24.Nb3 Ke7 25.Rd4 Bg6 26.c4 Rc6 27.Nxc5 Rxc5
  28.Rxa6 Rb8 29.Rd1 Rb2 30.Ra7+ Kf6 31.Ra1 Rf5 32.f3 Re5 33.Ra3
  Rc2 34.Rb3 Ra5 35.a4 Ke7 36.Rb5 Ra7 37.a5 Kd6 38.a6 Kc7 39.c5
  Rc3 40.Raa5 Rc1 41.Rb3 Kc6 42.Rb6+ Kc7 43.Kf2 Rc2+ 44.Ke3 Rxc5
  45.Rb7+ 1-0
  </code></pre>
  <p><a href="https://www.chessgames.com/perl/chessgame?gid=1431697" rel="noreferrer">https://www.chessgames.com/perl/chessgame?gid=1431697</a></p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T19:06:27.547Z
Answers.Text: +1. 7...h6 is a good old one I forgot.
Answers.UserDisplayName: Adhvaitha
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-08T04:28:54.077Z
Answers.Text: It's hilarious that Kasparov accused the computer of cheating using a human - how times have changed!
Answers.UserDisplayName: BlueRaja - Danny Pflughoeft
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T18:51:01.123Z
Answers.LastActivityDate: 2020-09-08T06:35:26.713Z
Answers.LastEditDate: 2020-09-08T06:35:26.713Z
Answers.UserDisplayName: Akavall
Answers.Body: <p>In Game 5 of Karpov vs. Korchnoi for the World Championship in 1978, Korchnoi missed a forced mate when he played Be4+ instead of Bf7+ on move 55. The game ended as a lengthy draw.</p>
  <p><a href="https://www.chessgames.com/perl/chessgame?gid=1068051" rel="nofollow noreferrer">https://www.chessgames.com/perl/chessgame?gid=1068051</a></p>
  <p>The match was being played as first to 6 wins with draws not counting, and neither player had won a game out of the first four. The match ended as a 6-5 win for Karpov in 32 games after Korchnoi nearly came back from the 5-2 deficit he faced after 27 games. Korchnoi won games 28, 29, and 31, and drew game 30, tying the match at 5-5 before Karpov won Game 32 to retain his title. Had Korchnoi won Game 5 and every following game played out the same, he would only have been down 5-3 following game 27 and the following three victories would have won Korchnoi the match in 31 games.</p>
  <p>This was a dramatic match that had bizarre allegations of cheating, as Guest_On_StackExchange mentioned in the comments to the original post. Additionally, this match had a similar dynamic to the match friscodelrosario mentioned, with Korchnoi being a Soviet defector challenging a Soviet champion. Korchnoi winning could have fascinating political/propaganda impacts on your alternate timeline.</p>
  <p>Details of the forced win:
  55. Bf7+
  55.. Kc6 (forced)
  56. Qe6+
  56.. Kb5 or Kb7</p>
  <p>If Kb5:
  57. Qc4+
  57.. Ka4 (forced)
  58. Qa6#</p>
  <p>If Kb7:
  57. Qxe7+
  Black can play Nd7, Kc8, Kc6, Ka6, or Ka8, but all are lost</p>
  <p>If Nd7 then Qxd7+ and Ka8 Qc8# or Ka6 Bc4+ b5 Qxb5#</p>
  <p>If Kc8 then Qc7#</p>
  <p>If Kc6 then Qc7+ Kb5 Bc4+ Ka4 Qxa7#</p>
  <p>If Ka6 then Bc4+ b5 Qd6+ Kb7 Qb8+ and Ka6 Bxb5# or Kc6 Qc7#</p>
  <p>If Ka8 then Qd8+ Kb7 Qc7+ and Ka8 Qb8# or Ka6 Bc4+ b5 Qc6#</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-11T00:08:19.087Z
Answers.LastActivityDate: 2020-09-11T00:08:19.087Z
Answers.UserDisplayName: Dylan Hayes
Answers.Body: <p>Unfortunately this example does not come from the 1985 match, but Kasparov arrived at the final game of the 1987 rematch needing to secure a win. He made an unusual choice by going for the English Opening for the first time ever against Karpov, ending in a great success. The game can be seen at <a href="https://www.chessgames.com/perl/chessgame?gid=1067242" rel="nofollow noreferrer">https://www.chessgames.com/perl/chessgame?gid=1067242</a></p>
  <p>Nobody will ever know what could've happened had Kasparov tried something else, but it'd be interesting to see an alternative timeline where the &quot;old guard&quot; player defeats the &quot;perestroika guy&quot;</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-10T21:02:11.583Z
Answers.LastActivityDate: 2020-09-10T21:02:11.583Z
Answers.UserDisplayName: David
Answers.Body: <p>If you don't mind going a bit earlier, then it has to be 1972 Spassky–Fischer (mother of all World Championships) Game 1.</p>
  <p>Fischer played 29...Bxh2? a move that few players would consider in light of the obvious 30.g3, trapping the bishop.</p>
  <p><strong>EDIT</strong> Later analysis (Speelman 1981) showed that the game is not lost (and can be drawn with extremely precise play), though the move was sub-optimal and the line becomes really sharp after 30. g3. The real blunder happened on move 37 with 37...Ke4 instead the optimal move again was just 37...a6. However, going by principles and elementary tactics 29...Bxh2? was an unnecessary move and chances are Fischer overlooked the fact that the trapped Bishop couldn't be saved.</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T18:30:58.617Z
Answers.Text: Thank you! May I abuse your goodwill a little and ask what would be the better move for Fischer?
Answers.UserDisplayName: JCCyC
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T18:38:39.723Z
Answers.Text: Well, almost every other move :-) 29...a6 is the most preferred one by the engine, simply exchange pawns and with both having bishops and equal number of pawns is a dead draw. Fischer tried to be a bit brave but failed miserably and made a move that even club players won't.
Answers.UserDisplayName: Adhvaitha
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T18:38:59.020Z
Answers.Text: Never mind, I found a site with commentary: https://www.chessgames.com/perl/chessgame?gid=1044722&comp=1 - says 29...a6. I'm going with that.
Answers.UserDisplayName: JCCyC
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T18:40:49.300Z
Answers.Text: You can also look at this documentary: https://youtu.be/lwLM9bgE3tY?t=1635 The game 1 is talked about for a bit around 27:30
Answers.UserDisplayName: Adhvaitha
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T19:29:12.950Z
Answers.Text: I'd like to credit you with your full name when (if) my book gets out. Can I? Or should I just say "Adhvaitha from StackExchange?"
Answers.UserDisplayName: JCCyC
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-08T07:06:24.490Z
Answers.Text: @JCCyC You can just quote the stackexchange page and my name as "Adhvaitha"
Answers.UserDisplayName: Adhvaitha
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-09-07T18:28:16.910Z
Answers.LastActivityDate: 2020-09-08T07:14:16.400Z
Answers.LastEditDate: 2020-09-08T07:14:16.400Z
Answers.UserDisplayName: Adhvaitha
Body: <p>Full disclosure: I'm not a regular chess player. This question is because I'm writing a book about alternate timelines, and I want to use a game in a real life famous chess match as a point of divergence. I'd like an occasion in which a sizeable portion of pundits and specialists thought a different move should have been made, and the player who made that move lost the game, or squandered an allegedly easy win for a draw.</p>
  <p>It'd be great if it was in the 1984/85 Karpov/Kasparov championship match, but any headlines-making match at any point in history would do. It could be Kasparov vs. Deep Blue, or Karpov vs. Korchnoi, for example.</p>
  <p>If I use your suggestion, I promise I'll properly credit you. <em>And</em> the players involved. Even if it's a computer.</p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2020-09-08T06:42:17.207Z
Comments.Text: "made that movie"?
Comments.UserDisplayName: user21820
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2020-09-08T14:21:28.670Z
Comments.Text: Thank you for caring enough about your work to do the proper research on the game.  Other examples are the chess game in the first Harry Potter movie, and the first episode of Code GEASS, (even though they're both mostly ignored).
Comments.UserDisplayName: Adam Barnes
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2020-09-10T12:24:21.980Z
Comments.Text: From a story rather than a chess point of view, Kasparov vs. Deep Blue turning out differently could conceivably have altered the trajectory of AI. For example, a convincing Kasparov victory might have led to the discovery of deep learning earlier by convincing researchers that Deep Blue was a dead-end and that if they wanted to beat a world champion then they deeded a fundamentally different approach.
Comments.UserDisplayName: John Coleman
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2020-09-10T20:33:32.397Z
Comments.Text: I'm not a chess expert either, but Korchnoi-Karpov was filled with drama every time. Allegations ranging from Soviets hiring a shrink to stare at Korchnoi while he played to codes hidden in a blueberry yogurt (some of the allegations were not serious, but some had alarming amounts of evidence) abounded. You can check out the gist on Korchnoi's biography on Wikipedia (https://en.wikipedia.org/wiki/Viktor_Korchnoi#First_World_Championship_match_against_Karpov).
Comments.UserDisplayName: Lige
Tags: <kasparov><karpov><fiction>
Title: Example of a Karpov vs. Kasparov game with a controversial move
ThreadText: # Example of a Karpov vs. Kasparov game with a controversial move
  
  (Asked by: username_0 on 2020-09-07)
  
  Full disclosure: I'm not a regular chess player. This question is because I'm writing a book about alternate timelines, and I want to use a game in a real life famous chess match as a point of divergence. I'd like an occasion in which a sizeable portion of pundits and specialists thought a different move should have been made, and the player who made that move lost the game, or squandered an allegedly easy win for a draw.
  
  It'd be great if it was in the 1984/85 Karpov/Kasparov championship match, but any headlines-making match at any point in history would do. It could be Kasparov vs. Deep Blue, or Karpov vs. Korchnoi, for example.
  
  If I use your suggestion, I promise I'll properly credit you. *And* the players involved. Even if it's a computer.
  
  **Tags:** kasparov, karpov, fiction
  
  > **Comments:**
  > **username_1**: "made that movie"?  
  > **username_2**: Thank you for caring enough about your work to do the proper research on the game.  Other examples are the chess game in the first Harry Potter movie, and the first episode of Code GEASS, (even though they're both mostly ignored).  
  > **username_3**: From a story rather than a chess point of view, Kasparov vs. Deep Blue turning out differently could conceivably have altered the trajectory of AI. For example, a convincing Kasparov victory might have led to the discovery of deep learning earlier by convincing researchers that Deep Blue was a dead-end and that if they wanted to beat a world champion then they deeded a fundamentally different approach.  
  > **username_4**: I'm not a chess expert either, but Korchnoi-Karpov was filled with drama every time. Allegations ranging from Soviets hiring a shrink to stare at Korchnoi while he played to codes hidden in a blueberry yogurt (some of the allegations were not serious, but some had alarming amounts of evidence) abounded. You can check out the gist on Korchnoi's biography on Wikipedia (https://en.wikipedia.org/wiki/Viktor_Korchnoi#First_World_Championship_match_against_Karpov).  
  
  ---
  
  ## 6 answers
  
  ### Answer by username_8 on 2020-09-07. Score: 20 | Accepted answer
  
  If you don't mind going a bit earlier, then it has to be 1972 Spassky–Fischer (mother of all World Championships) Game 1.
  
  Fischer played 29...Bxh2? a move that few players would consider in light of the obvious 30.g3, trapping the bishop.
  
  **EDIT** Later analysis (Speelman 1981) showed that the game is not lost (and can be drawn with extremely precise play), though the move was sub-optimal and the line becomes really sharp after 30. g3. The real blunder happened on move 37 with 37...Ke4 instead the optimal move again was just 37...a6. However, going by principles and elementary tactics 29...Bxh2? was an unnecessary move and chances are Fischer overlooked the fact that the trapped Bishop couldn't be saved.
  
  > **Comments:**
  > **username_0**: Thank you! May I abuse your goodwill a little and ask what would be the better move for Fischer?  
  > **username_8**: Well, almost every other move :-) 29...a6 is the most preferred one by the engine, simply exchange pawns and with both having bishops and equal number of pawns is a dead draw. Fischer tried to be a bit brave but failed miserably and made a move that even club players won't.  
  > **username_0**: Never mind, I found a site with commentary: https://www.chessgames.com/perl/chessgame?gid=1044722&comp=1 - says 29...a6. I'm going with that.  
  > **username_8**: You can also look at this documentary: https://youtu.be/lwLM9bgE3tY?t=1635 The game 1 is talked about for a bit around 27:30  
  > **username_0**: I'd like to credit you with your full name when (if) my book gets out. Can I? Or should I just say "username_8 from StackExchange?"  
  > **username_8**: @username_0 You can just quote the stackexchange page and my name as "username_8"  
  
  ---
  
  ### Answer by username_6 on 2020-09-07. Score: 17
  
  Kasparov's 7...h6? in the game below cost him the game and the match, and later Kasparov accused Deep Blue team of cheating.
  [FEN ""]
  [Event "IBM Man-Machine"]
  [Site "New York, NY USA"]
  [Date "1997.05.??"]
  [EventDate "?"]
  [Round "6"]
  [Result "1-0"]
  [White "Deep Blue (Computer)"]
  [Black "Garry Kasparov"]
  [ECO "B17"]
  [WhiteElo "?"]
  [BlackElo "?"]
  [PlyCount "37"]
  
  1.e4 c6 2.d4 d5 3.Nc3 dxe4 4.Nxe4 Nd7 5.Ng5 Ngf6 6.Bd3 e6
  7.N1f3 h6 8.Nxe6 Qe7 9.O-O fxe6 10.Bg6+ Kd8 11.Bf4 b5 12.a4
  Bb7 13.Re1 Nd5 14.Bg3 Kc8 15.axb5 cxb5 16.Qd3 Bc6 17.Bf5 exf5
  18.Rxe7 Bxe7 19.c4 1-0
  
  https://www.chessgames.com/perl/chessgame?gid=1070917
  I would also like to suggest a move from a final game of the tie-break in Kramnik - Topalov from classical-Fide championship unification match in 2006. With 44...Rxc5 Topalov went from difficult position to instantly losing. The match was full of controversy if this is what you are looking for.
  [FEN ""]
  [Event "Kramnik - Topalov World Championship Match"]
  [Site "Elista RUS"]
  [Date "2006.10.13"]
  [EventDate "?"]
  [Round "16"]
  [Result "1-0"]
  [White "Vladimir Kramnik"]
  [Black "Veselin Topalov"]
  [ECO "D47"]
  [WhiteElo "?"]
  [BlackElo "?"]
  [PlyCount "89"]
  
  1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 e6 5.e3 Nbd7 6.Bd3 dxc4 7.Bxc4
  b5 8.Be2 Bb7 9.O-O Be7 10.e4 b4 11.e5 bxc3 12.exf6 Bxf6
  13.bxc3 c5 14.dxc5 Nxc5 15.Bb5+ Kf8 16.Qxd8+ Rxd8 17.Ba3 Rc8
  18.Nd4 Be7 19.Rfd1 a6 20.Bf1 Na4 21.Rab1 Be4 22.Rb3 Bxa3
  23.Rxa3 Nc5 24.Nb3 Ke7 25.Rd4 Bg6 26.c4 Rc6 27.Nxc5 Rxc5
  28.Rxa6 Rb8 29.Rd1 Rb2 30.Ra7+ Kf6 31.Ra1 Rf5 32.f3 Re5 33.Ra3
  Rc2 34.Rb3 Ra5 35.a4 Ke7 36.Rb5 Ra7 37.a5 Kd6 38.a6 Kc7 39.c5
  Rc3 40.Raa5 Rc1 41.Rb3 Kc6 42.Rb6+ Kc7 43.Kf2 Rc2+ 44.Ke3 Rxc5
  45.Rb7+ 1-0
  
  https://www.chessgames.com/perl/chessgame?gid=1431697
  
  > **Comments:**
  > **username_8**: +1. 7...h6 is a good old one I forgot.  
  > **username_9**: It's hilarious that Kasparov accused the computer of cheating using a human - how times have changed!  
  
  ---
  
  ### Answer by username_5 on 2020-09-07. Score: 11
  
  Not exactly what you're looking for, but in his 1987 match vs Karpov, Kasparov forgot to press his clock during time pressure. Kasparov finally lost that game, the result being heavily influenced by the clock's incident.
  
  <https://www.latimes.com/archives/la-xpm-1987-10-15-mn-14340-story.html>
  
  <https://apnews.com/d00b24ebff5eede461c2587317bcef62>
  
  That a professional chess player forgets to press his clock after making a move is quite an extraordinary event.
  
  > **Comments:**
  > **username_0**: Paywall. Needs to be a subscriber, and they don't have a free registration. :(  
  > **username_5**: Hm, that's weird, since I don't have an account or anything like that. Myabe I can access it because I'm from out the US? Ok, try this one; https://www.csmonitor.com/1987/1104/lchs04.html If that one doesn't work either, try googling for "karpov kasparov 1987 clock" or something similar.  
  > **username_6**: I was able to access both, and I am in the US.  
  > **username_0**: Read the AP article. Jeez. Karpov is a dick. Or not. Maybe he DID make that "mistake" of "inadvertently" warning Kasparov on purpose. We'll never know.  
  
  ---
  
  ### Answer by username_7 on 2020-09-08. Score: 11
  
  We'll never know the whole truth of the 1951 world championship match. Years after the Soviet Union collapsed, and nearing death, Bronstein still wouldn't talk about it.
  The challenger Bronstein was the son of an imprisoned Jewish dissident, while champion Botvinnik was a model Soviet (dichotomy much the same as Korchnoi and Karpov in the '80s). In the memoir The Sorceror's Apprentice, all Bronstein would say was that he was "under great psychological pressure, and it was up to [him] how [he] dealt with it."
  We've speculated over 57. Kc2?? in Game 6 for decades, and whether Bronstein resigned prematurely in Game 23 is also a source of doubt.
  I don't think Bronstein felt he was permitted to win that match. And he didn't lose it, he tied it. Like he knew all along he could've taken the title away if he thought he and his family could survive that.
  rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
  [Event "Botvinnik - Bronstein World Championship Match"]
  [Site "Moscow URS"]
  [Date "1951.03.26"]
  [Round "6"]
  [Result "0-1"]
  [White "username_11 Bronstein"]
  [Black "Mikhail Botvinnik"]
  
  1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 d6 6.Bg5 e6 7.Qd2
  h6 8.Bxf6 gxf6 9.O-O-O a6 10.f4 Bd7 11.Kb1 Be7 12.Be2 Nxd4
  13.Qxd4 Qa5 14.Rhf1 h5 15.Rf3 Qc5 16.Qd2 Bc6 17.Re3 Qa5 18.Bf3
  O-O-O 19.Qd3 Rd7 20.h4 Kb8 21.a3 Bd8 22.Ka2 Qc5 23.Re2 a5
  24.a4 Bb6 25.b3 Rc8 26.Qc4 Qxc4 27.bxc4 Rh8 28.Kb3 Rdd8 29.Rd3
  Bg1 30.Red2 Kc7 31.Ne2 Bf2 32.Rd1 Bc5 33.Ng3 Rdg8 34.Ne2 Rh7
  35.f5 e5 36.Nc3 Bd4 37.Rxd4 exd4 38.Rxd4 Rhg7 39.Ne2 Rxg2
  40.Bxg2 Rxg2 41.Nf4 Rg3+ 42.Kb2 Rg4 43.Nxh5 Rxh4 44.Nxf6 Kb6
  45.Rxd6 Kc5 46.e5 Rd4 47.Rxd4 Kxd4 48.Ng4 Bxa4 49.e6 fxe6
  50.f6 Be8 51.Kb3 e5 52.c3 Ke4 53.Nh6 Kf4 54.f7 Bxf7 55.Nxf7 e4
  56.Nd8 e3 57.Kc2?? Kg3 0-1
  
  
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  ### Answer by username_10 on 2020-09-11. Score: 2
  
  In Game 5 of Karpov vs. Korchnoi for the World Championship in 1978, Korchnoi missed a forced mate when he played Be4+ instead of Bf7+ on move 55. The game ended as a lengthy draw.
  
  <https://www.chessgames.com/perl/chessgame?gid=1068051>
  
  The match was being played as first to 6 wins with draws not counting, and neither player had won a game out of the first four. The match ended as a 6-5 win for Karpov in 32 games after Korchnoi nearly came back from the 5-2 deficit he faced after 27 games. Korchnoi won games 28, 29, and 31, and drew game 30, tying the match at 5-5 before Karpov won Game 32 to retain his title. Had Korchnoi won Game 5 and every following game played out the same, he would only have been down 5-3 following game 27 and the following three victories would have won Korchnoi the match in 31 games.
  
  This was a dramatic match that had bizarre allegations of cheating, as Guest\_On\_StackExchange mentioned in the comments to the original post. Additionally, this match had a similar dynamic to the match username_7 mentioned, with Korchnoi being a Soviet defector challenging a Soviet champion. Korchnoi winning could have fascinating political/propaganda impacts on your alternate timeline.
  
  Details of the forced win:
  55. Bf7+
  55.. Kc6 (forced)
  56. Qe6+
  56.. Kb5 or Kb7
  
  If Kb5:
  57. Qc4+
  57.. Ka4 (forced)
  58. Qa6#
  
  If Kb7:
  57. Qxe7+
  Black can play Nd7, Kc8, Kc6, Ka6, or Ka8, but all are lost
  
  If Nd7 then Qxd7+ and Ka8 Qc8# or Ka6 Bc4+ b5 Qxb5#
  
  If Kc8 then Qc7#
  
  If Kc6 then Qc7+ Kb5 Bc4+ Ka4 Qxa7#
  
  If Ka6 then Bc4+ b5 Qd6+ Kb7 Qb8+ and Ka6 Bxb5# or Kc6 Qc7#
  
  If Ka8 then Qd8+ Kb7 Qc7+ and Ka8 Qb8# or Ka6 Bc4+ b5 Qc6#
  
  
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  ### Answer by username_11 on 2020-09-10. Score: 1
  
  Unfortunately this example does not come from the 1985 match, but Kasparov arrived at the final game of the 1987 rematch needing to secure a win. He made an unusual choice by going for the English Opening for the first time ever against Karpov, ending in a great success. The game can be seen at <https://www.chessgames.com/perl/chessgame?gid=1067242>
  
  Nobody will ever know what could've happened had Kasparov tried something else, but it'd be interesting to see an alternative timeline where the "old guard" player defeats the "perestroika guy"
  
  
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