I was talking with a friend about the period at the end of Burning Crusade when the crates began spawning in major cities. He reminded me that neither of us had really paid much attention - his guild was in the process of breaking up over M'uru/Entropius and I was tanking for a guild working on clearing Black Temple at the time. We were busy, is what I'm saying. So busy, in fact, that one day we found ourselves running for our lives from an Ironforge that was completely infested with the walking dead. Other players were now zombies. The auctioneers were dead. It was all chaos and madness.
Now, for a lot of people, the zombie invasion was a load of fun. It was new and different, something you didn't see in game every day. Some of my guildies went over to Orgrimmar and joined forces with Horde players they knew (Norgannon was a smallish and incestuous server in those days, all the Alliance and Horde players seemed to know one another) to form roving gangs of undead, laying waste to all, Horde or Alliance. They had great fun.
For me it was a huge pain in the rear end.