Those familiar with World of Warcraft have probably heard of the legendary accidental event known as the Corrupted Blood Incident, where back in 2005 a highly-contagious lethal debuff managed to spread outside of the raid instance it originated from with the help of teleportation, resulting in a digital pandemic that included programmer-imposed quarantine zones and a mass exodus of players from highly populated cities. It was spectacular, the sort of thing that almost never succeeds out of deliberate intent.
An “event” of a different sort with echoes of that incident hit Azeroth recently, though this was the result of hacking rather than game mechanics gone awry. It seems somebody managed to find a rather serious exploit that allowed them to fly around as a level 1 priest, effortlessly killing anything they came in contact with.
The massacre occurred over several hours, piling Stormwind high with bodies, forcing bewildered players to evacuate to escape the griefing, which soon was taking place across several US and EU servers.
The hack was developed by a small group of players going by Jadd, Master674 and Cencil, and seems to work on NPCs as well as player characters. After one of their accounts was banned while testing their creation, they decided to share it with some friends, knowing that Blizzard would be hotfixing it soon after.
While this is hardly on the same level as the organic random event that came out of the Corrupted Blood Incident, it is without a doubt one of the most widespread MMO hacking incidents in history, and probably the most interesting thing to happen in WoW in the last seven years.