Worlds of Warcraft, that online game that has the largest subscription base on earth, experienced a "bug" involving a plague-like epidemic. Granted, they didn't mean to have it happen, but the circumstances seem intriguing:

Some servers have gotten so bad that you can't go into the major cities without getting the plague (and anyone less than like level 50 nearly immediately die). GM's even tried quarantining players in certain areas, but the players kept escaping the quarantine and infecting other players.

A funny and frustrating mistake - but one that they may try to work with in the future? I know a storyline where folks were sick all the time would be pretty compelling, I think. I haven't played video games in a while, but I know that this sort of "experience" is what most people want when playing those Massive Multi-player things. Something that shows the world is alive and has rhythm...What else could make an equally compelling event?