In early Runes of Magic there was an event where an event robbed a bank. It was supposed to despawn before it died, but there was a rogue ability that made everything crit for 10 seconds(10 min cd). I was able to kill it with that ability and completely bug the event. The banker (that despawned during the event) was gone until they patched the servers a few days later.

At GW2 launch hundreds of players were stuck inside the cities for like an hour.