Player receives Developer item in the mail, one-shots Ulduar
TL;DR: A GM sent a player a GM item with additional effect AoE Death. Player uses item to completely clear new raids. Blizzard bans everyone in the guild, even those that hadn't been on any of the raids.
There are of course differences between this and the Salvage duping bans, but Blizzard was able to find out about this because the person got a lot of first Achievements which made it pretty obvious what they were doing. Oh and it recorded their highest damage hit of something like 350 million damage (the max HP of one of the hardest hard mode new bosses). If they had been smarter they could have waited and abused the hell out of it.
This isn't to start any kind of WoW v. FFXI war or anything. I just thought it was interesting to get a perspective on what other MMO's do when something hinky happens.