This happened a couple of months ago - someone managed to train Grol The Destroyer into Stormwind to see the King, whereupon he wreaked havoc upon hundreds of players as they tried to kill him. I wandered over from Ironforge to see what all the fuss was about, fired a few arrows at him and THEN realised that only two days beforehand, I'd completed the precursor quest to obtain the
Enchanted Azsharite Felbane Sword and still had it in my bag - after a small "OMG OMG" moment and waiting for the cooldown on equipping it, one strike and he was down.
Awesome
I don't think anyone else knew about needing the Felbane weaponry to take him down, it's a very cool chain that starts with the Fallen Hero of The Horde in the Swamp of Sorrows (the one that has you picking up Giant poo!).