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