In chess, a pawn may be captured “in passing” — when a pawn advances two squares from its initial position, it may be captured by an adjacent pawn as if it had advanced only one square.

This can lead to a curious state of affairs:

fraenkel en passant chess problem

From this position White plays 1. Bg2+ and declares checkmate. Black says “Au contraire,” plays 1. … d5, and announces checkmate himself. White shakes his head, plays 2. cxd6 e.p., and reasserts his own claim:

fraenkel en passant chess problem

Black claims that this last move is absurd. He says the game ended when he advanced his pawn to d5. But White argues that the pawn never reached d5 — in principle it was captured on d6, and thus could not stop White’s original mate.

So who won the game? It would seem to be a matter of opinion!

From Heinrich Fraenkel, Adventure in Chess, 1951.