I don't believe it can be done deliberately. It has happened before, I believe it's a minor bug in the programming. If a player moves a pawn to row 8, then submits the move, a promotion screen is offered to choose queen or another piece. If the player then goes to another game without choosing anything the move is never completed. Other players can't fix it, only the site owner's staff can, so you have to SEND FEEDBACK if you want it fixed.