It's called en passant:
https://support.chess.com/customer/en/portal/articles/1444779-my-opponent-made-an-illegal-move-
Pawn Cheat
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It appears that the pawn has special abilities. For example i moved Pawn (black) from f2 to f4 then pawn (white) at e4 takes my pwn at f4 and moves to square f3...
Proof
http://www.chess.com/livechess/game?id=1373674931
moves 20 and 21.
This has happened to me in 3 seperate games and if it helps it usually occurs at the bottom corner of the game from f-h 4< squares.
Thank you