I was phoned at my house one night to go along to Sandy Bells
and stop a riot over a row about the En Passant rule.
I had to carefully explain to this brute of man what the player did was legal.
DO NOT PLAY EN PASSANT IN A PUB AGAINST A STRANGER!
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Remember that post from about 10 years ago when both players
thought it was checkmate (it had been with a pawn and it was not
checkmate because the mated played could play PxP en passant.)
We tried dropping hints but both players thought one had checkmated the other.
They blamed the RHP computer and things started getting nasty when they were
told to study the rules of chess.
Then a MOD happened along to tell us to stop discussing a game in progress.
The rule that causes the most controversy in our league matches is the condition that a win on time only counts if the winning player has sufficient mating material, otherwise it's a draw. This doesn't mean with best play - it just means that a mate is technically possible with the material on the board. I've seen several arguments over that!