Mound mistake' refers to a baseball term used to describe an illegal move made by the pitcher. It is when the pitcher moves his pivot foot off the mound before throwing the ball, which is against the rules of the game. The term comes from the fact that the pitcher must have both feet on the mound in order to throw the ball legally. The solution to the crossword clue 'BALK' is a four-letter word that refers to this illegal move made by the pitcher.