Especially in blitz or bullet games, it is possible that a player makes an illegal move, for example castles through check. 

* If the opponent does notice the illegal move, then as far as I know the first player is obliged to make a legal move with the same piece, if one exists. What if there are no legal moves? 
* What if the opponent, being in time pressure, doesn't notice the illegal move and makes a move. What happens after they realize that? Does the person who made the illegal move forfeit? Or is the whole game cancelled?

Are there any standard rules for these kinds of situations?
## Answer (6)

There are, indeed, rules regarding these situations. I'll provide some references, where you can find more detail.

Rule 4.1 on the official FIDE website states: *"If a player has made an illegal move, the game is stopped and the opponent may decide to continue the game or claim a win by threefold repetition, the touch-move rule or 50-move rule or request a recapture. The
## Answer (4)

1. A move that is clearly illegal is an invalid move.

* Your move has no effect unless it also is illegal for your opponent to respond.
* If there is a legal move your opponent must play it.
* And in some situation the opponent might not be able to play a legal move for his next turn, in particular in cases involving 3-check, repetition, or double check.

In those case,