If you accidentally put yourself in checkmate in a game of chess, it's considered an illegal move.
Here's a breakdown of what generally happens:
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Illegal Move: The move that places your own king in checkmate is against the rules of chess.
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Move Retraction: The illegal move must be retracted. You take back the move that put yourself in checkmate.
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New Move Required: According to the "touch-move" rule, if possible, you must then make a legal move with the piece you touched. If moving that piece legally isn't possible, you can make any other legal move. This assumes you have actually touched the piece during a physical game. In online chess, this is usually automated; you simply have to make any legal move.
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Time Penalties (Classical Chess): In some formal, timed classical chess settings, your opponent may be awarded extra time on the clock (e.g., an additional two minutes). This is to compensate them for the disruption and having to consider a move that wasn't actually legal. This doesn't apply to casual games or most online scenarios.
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No Loss of Game: You don't automatically lose the game for making an illegal move that puts yourself in checkmate (unless the rules of a specific tournament state otherwise, which is rare).
Important Considerations:
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Touch-Move Rule: The "touch-move" rule is crucial. It states that if you deliberately touch one of your pieces during your turn, you must move that piece if it's legal to do so. If you touch an opponent's piece, you must capture it if a legal capture is possible. This rule is typically enforced in over-the-board (physical) chess.
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Online Chess: In online chess, the move is usually rejected immediately, and you are prompted to make a legal move. The "touch-move" rule is typically less relevant because you haven't physically touched a piece.
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Arbiters: In formal tournaments, an arbiter (a chess official) is present to enforce the rules and make rulings in case of disputes.
In summary, accidentally checkmating yourself is a mistake that results in the move being taken back, and you are required to make a legal move (preferably with the same piece, if possible), potentially with a time penalty added to your opponent's clock in classical play.
