When the first move is an en passant the replayer plays this successfully, but on going backwards it loses the knowledge of the en passant and creates a piece, in this case a bishop, on the capturing square.

Try scrolling forwards and then backwards in this example.

[title "Third part - mate in 1"]
[fen "5R2/p2k1p1P/1P1P1PPr/bPpKB2p/1p4n1/7B/2P2N1P/1b6 w KQkq - 0 1"]

1. bxc6#

This bug is also prevalent when you delay the en passant capture.

[FEN ""]

1. e4 e6 2. e5 d5 3. d4 c5 4. exd6
  • Three small comments: 1. In the first FEN, the "en passant square" is missing from the FEN. The "-" after "KQkq" should be replaced by "c6". Unfortunately, this does not seem to fix the bug. 2. The en passant move is also not replayed successfully, as the captured Black pawn does not disappear from the board. 3. "Delaying the en passant capture" is not legal, right? The replayer seems to only allow en passant when the last move was a double pawn move, which is what the rules say. But it should be at least fixed to also take the FEN's "en passant square" into account. – wimi Dec 22 '20 at 8:35

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