I was once playing chess in a city park when I sacrificed a piece and realised, a moment too late, that the sacrifice was flawed. My opponent saw what I thought I had seen and resigned. I said, "Don't resign, I made a mistake. You can win from there."
He examined the board for five minutes and insisted that he had lost so I changed sides with him and played his position.
It's the only time I've ever won the same game twice with opposite coloured pieces.
He found THAT very annoying.