Game of Diplomacy. Only four surviving players left. Leader has sixteen Supply Centers while second strongest only has six. The leader enters Civil Disorder and disconnects from the game. Just as we're all about to call a draw, someone else joins and takes over the leader's territory, claims two more supply centers, and wins the game without doing a single thing to earn it.
Dammit but I want to find out who that guy is and strangle him. Or leave a bag of burning dog crap at his front door. You know, one or the other.