I have a couple that make me giggle. The first is accidental. Back in BC, my guild was doing Magetheridon and we were setting up in the bosses room. Well, for Mag, the fight doesn't start untill you attack the mobs, no face pulls. So we were all getting buffed up and in positions, when my hand slips and I hit sinister strike on the mob, starting the pull and decimating the raid. I thought it was funny, but I was pretty much alone in that thought.
Lately, in slow pugs that take forever to get started, me and a friend play a game of chicken with boss aggro range. Whoever can get closest without dying wins. If you don't want to pull, you can chicken out but you lose. Well, we were playing, and....let's just say I know where Onyxia pulls from now. In the same raid, after wiping a few times and being on a 10 minute AFK break, we were messing around on the side of the bridge right before Ony, which just happens to be above a whelp cave. Me and my priest friend start joking around in /say "oh yea, it's so sweet that you don't pull aggro and spawn whelps when you jump down there with levitate on, since you don't hit the ground." After discussing that for a couple mins, my friend starts levitating people. Shortly thereafter, a ret pally jumps down and spawns a bunch of whelps. Unsatisfied with this, he decides to bubble and try to pull the whelps back to us. Somehow, he ran and aggro'd Ony as well. It was a good time, and somehow that group managed to get Ony down.