I was playing in a tournament recently and something rather unusual happened - both were getting short of time (but not a total time scramble yet) and one player captured a piece that put his opponent in check, but neither player realized it. The other player immediately played a move to deliver “checkmate”, and they both agreed the game was over. A few minutes later, with both players still present but not at the board, another player walked past the board still in the final position, and asked the players how both kings could be in check. The TD was brought over but unsure of how to rule, and the players then agreed to a draw, but after reflection (and after the tournament was over) the TD let the “win” stand since the players had originally agreed to the outcome.
That sounds like the right decision to me, I’m curious what other ideas might be…