Wimbledon is the only tennis grand slam that doesn't have a final set tie break. This means that there is no maximum number of games in the 5th set (for men). Famously John Isner and Nicolas Mahut played a 138 game 5th set in 2010.
Without the final set tie break, matches can last far longer than anticipated, causing scheduling problems and leaving the winner of such marathon games physically exhausted for the next game. So why doesn't Wimbledon have the tie break?
EDIT: In 2018 Wimbledon introduced added a tie break to the 5th set. If players reach a 12-12 games tie, a tie breaker will be played to determine the winner of the match. In its first year, this rule came into effect in the finals match for the very first time.