Last night in Heroic Magister's Terrace, I accidentally ninjaed a Cudgel of Consecration. I'm not sure what was going through my head -- I'm currently a Holy priest, and clearly need it much less than the Prot pally tank I was grouped with. Somehow I'd gotten under the impression that it was a healing item, or something. I was, understandably, promptly called a noob and yelled at by several members of the group, and there are a couple hunters out there who may well never group with me again. I profusely apologized to the tank, and we both opened tickets and sat around in the instance waiting for them to get answered. (The response, un-helpfully enough, was "your ticket has been escalated; this may take several days to be resolved.")
I gave him the mats for a Major Spellpower enchant, to put on the Cudgel once he gets it, to help make up for my stupidity. We parted on pretty good terms; we're hopeful that the GMs will sort this out. But picking up that item may well have been the biggest "d'oh" moment I've had in the game thus far. Either that, or accidentally taking a point in Throwing Weapon Specialization one time while respeccing my rogue. What's the most boneheaded mistake you've made in your WoW career?