All Fives: turn the muggins call on

In All Fives, agree before the first tile that a missed score is fair game: if a player completes a scoring play and does not call it, any opponent may say "muggins" and take those points instead. The call has to come before the next player lays a tile — after that the ends have changed and there is no longer an agreed score to claim. The call is optional and varies between rule sets, which is what makes turning it on a decision for the table rather than a change to the rules, and it is where the game's other name comes from.