If you game enough, you’re bound to run into someone being an ass. What’s the most asinine thing someone’s done in a game with you? How did you react? Did that experience change the way you game?
Almost gave up on the entire campaign (Spelljammer, done using a system cobbled together from Torg and Warhammer) after a severe derailing by a couple of players who decided that short term fun (taking down everybody else on a commercial ship) is better than a decent start to a long string of adventures.
Killed the whole party by introducing a timely elven naval party. Or would have, if one of the players hadn’t been holding an “escape” card (from Torg, for those keeping the score at home), rendering the whole encounter to a thing to be reminisced about.
Sadly, the campaign didn’t last much longer.
And the main instigator of the incident didn’t get invited to the next one.