WoW had quite a few noteworthy bugs throughout it's life time. One of the most famous was the "corrupted plaque" outbreak: (
http://www.wowwiki.com/Corrupted_Blood).
Another funny one was during the 1.9 patch on the PTR where someone discovered a bug involving a paladin talent that was allowing paladins to summon Kodo mounts after casting a spell called Judgement of Righteousness. It was so funny and very strange in how the heck Blizzard's code could be so screwed up to even allow such a completely unrelated effect of summoning a mount instead of a spell effect.
There were two other bugs during the closed beta for the TBC expansion that was quite amusing and frustrating for some players in the beta.
One of them involved accidental model swap with the Fel Reaver that roamed around in the Hellfire Peninsula zone. Instead of being a giant robot monster, it was replaced with a small gray bear! So many players were quite by surprised when they heard the sound of the fel reaver from far away only to see a small bear run up to them and kill them in one hit! It was nick named the "bear reaver" and many players would joke around making bear puns in the zone's chat.
The other bug had to do with a glitch with the female Naga model in the Zangermarsh zone. The glitch involved a display bug with the portraits of the female naga that if a player were to target one of them, the bug would cause the client to crash. Many testers avoided the zone for a while to avoid targeting these bugged mobs. Even if they didn't try to target them, if one of these mobs were accidentally aggroed, the player would automatically target them and BAM! instant crash.
The cake is a lie... THE CAKE IS A LIE!!!!!