Sorry...continued.
"How can you checkmate my queen," I said.
"No," he said. "That's your king."
I tapped the top of my piece. "Look, it has a crown on top."
"That's how you tell a king," he said. "Have you ever seen pictures of kings or movies of kings? Don't they always wear crowns?"
He had me there.
This happened a few weeks ago to Jonathan Speelman. He was playing a simul at Oxford University (with regular Staunton chess pieces on every board) and after about ten moves he checkmated one of the weaker players only to discover that he had checkmated his queen by mistake. His opponent promptly won Speelman's queen but was still not able to win the game (I think it was a draw, but Speelman definitely didn't lose).