Judit Polgár
In 1994 Judit suffered a controversial defeat at the hands of then-world champion Garry Kasparov, the highest-rated chessplayer of all time. Kasparov changed his mind after making a losing move and then made another move instead. According to chess rules, once a player has released a piece s/he cannot make a different move, so Kasparov should have been made to play his original move. However, Polgár did not challenge this because she says there were no witnesses and an arbiter was not around. She was also unaware at the time that the re-move was caught on tape by a television crew. The tournament director was criticised for not forfeiting Kasparov when the videotape evidence was made available to him.