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What happens if a player makes an illegal move (i.e. a realistic case could be, he castles after he already moved the King), but none of the player notices, and the game goes on and on for a lot of moves? Is the final result to be taken as valid? On another note, what happens if a player, after some moves, realizes that, say, move 10 was illegal, and could not be made?

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The FIDE laws of chess state:

If during a game it is found that an illegal move, including failing to meet the requirements of the promotion of a pawn or capturing the opponent’s king, has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined the game shall continue from the last identifiable position prior to the irregularity. The clocks shall be adjusted according to Article 6.13. The Articles 4.3 and 4.6 apply to the move replacing the illegal move. The game shall then continue from this reinstated position.

So basically the game should be reverted to before that move, using the game record.

Edit: These are for standard play. Other time controls have different rules, and although most national federations basically just apply the FIDE rules, they can sometimes be different.

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  • So if the game has ended, it should be repeated from that move?
    – A. N. Other
    Commented Oct 18, 2018 at 12:11
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    No. The rule applies only to games in progress (including adjudication), except I believe if the illegal move was the final move. It would be interesting to know if anybody has used this rule as a mulligan in case the game doesn't go their way, to be able to revert back to an earlier point.
    – Ananke
    Commented Oct 18, 2018 at 12:15
  • Interesting. Once in a tournament, I was in check, but neither of us noticed. Then, my opponent noticed and we didn't know what to do, so we called a staff over. The staff said once a move is done and the next person moves then you can't go back. This was repeated by my chess coach. We both had our notation. However, once, I made an illegal move in a chess tournament and my opponent saw and called the director over and I had to redo the move. He said there is a rule where 3 illegal moves and you lose. We also weren't playing with time.
    – Marvin
    Commented Oct 18, 2018 at 21:32

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