One square from a second queen
Round two, move 27. The pawn on c6 steps to c7, one more square and it turns into a queen, and I already had a queen on the board. So I stood up, walked to the neighbouring board and borrowed their spare one for psychological warfare, before he had even replied. It sat next to the pawn for the rest of the game, where he had to look at it.
The engine says Black was winning here by about three pawns and that the rook belonged on c5 instead. Nobody told me either of those things. I would have done it anyway. He cracked seven moves later.