Muggins: your opponent may claim what you missed

If a player announces a score lower than their cards are worth, the opponent calls "Muggins!" and pegs the missed points themselves. The call has to come after that player has finished pegging and before the next score is pegged or the hand is gathered up. Two exceptions: no muggins on the two points for a Jack starter once the dealer has played a card, and none when a player over-pegs rather than under-pegs. Agree it before the first deal — it is the one optional rule the American Cribbage Congress recognises, and it rewards counting your opponent's hand as carefully as your own.