This is pretty embarrassing but whatever.
the position is below. I was playing in a tournament and realized this was a completely lost position. Somehow, I determined that Rc8 was mate. I whispered, "omg, that's checkmate, and I played the move." my opponent looks like he got a heart attack. Until the guy next to me told me that's an illegal move because my rook is pinned to the king. He obviously thought I was an idiot. But my opponent's like "dude, you scared the crap out of me." I felt so bad lol but it was hilarious..and I'm so idiotic haha
well, this isent the most suprising thing that happened to me in an otb tournament, but one time i was playing in a team tournament and one of my teamates made an illegal move, and his opponent called the tournament director and said that making an illegal move should count as losing a game lol, then the coach on the team we were playing came over and they were arguing for a while lol.