The piece of paper that fooled Hitler
I from a crypto history book I read in high school: in WWI, German intercepts of Russian transmissions have the Russians illusions of German tactical genius.  Somehow, Germans would often be in position to confound the Russians.  The reason for this: Russian code clerks were badly trained.  Sometimes there would be a couple of attempts to send a message followed by a plain-text transmission by the frustrated code clerk.  This was a tremendous boon to German cryptographers in the long term as well as a giveaway of tactical maneuvers in the short term.In WWII, the Germans had their turn at incompetence.  Enigma machines were supposed to be "warmed up" by 3 random characters.  The purpose of this is like an Initial Vector or a "nonce" in modern block cipher use -- a bit of entropy to help obscure often almost identical message headers.  A lot of German code clerks were lazy and just typed in "AAA", which helped out the Bletchley Park cryptographers a lot.Modern crypto technology is often very good, but only if used correctly.  Security fails most often because of human error.  (This includes programmers!)