Steve and I play slow chess and the board is positioned on the coffee table in the living area.  We have several chess sets and Sam decided that he would like to play slow chess with me on one of the other sets.
I set the board up and he made first move.  We have a convention; a piece of wood is moved to the opponent's side of the board after a move is made.  I made my move later that evening.  Sam came to make his move while I was nearby, declaring “Check Mate!” “What?”, I said.  Sure enough it was “Check Mate”.  “Hang on a minute, that can't be right”, I insisted.  The little rascal had, when we counted the moves, done an extra move without moving the wood piece to my side.  Nice one Sam!