2019 April 02
originally posted on facebook

Chess site “lichess” does not properly validate chess games imported into the site in pgn notation. This permits odd behavior such as moving into check (but only if the king itself is moved) or promoting pawns into kings. If you have multiple kings and one of them is in check, sometimes the wrong one will be displayed as being checked. You can take your opponent’s king, but if it is their last king the website crashes. If a player has more than one king, or is in check on the opponent’s turn, it is considered “game over”, but if you capture the excess kings or leave check it returns to a non-game-over state.

After confusing the game like this, the engine will suggest crazy moves like moving the opponent’s pieces or taking its own pieces and seems to try to “helpmate” or something.

http://tom7.org/chess/cve.pdf

To test for yourself go to: https://lichess.org/analysis

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