I've seen a few strange bugs in WoW before.
One time the world server crashed while we were wiping on Kil'Jaeden (a bit sad seeing as we had him on farm for ages) and for some reason, instead of being teleported to the graveyard and disconnected when people released, their ghosts just spawned on top of their corpses and let them resurrect. This completely bugged out the boss script, because he just got stuck casting Soul Flay on a corpse endlessly and stopped using any other abilities, and his health appeared to be stuck too for some people. We just kept attacking him as we couldn't wipe or leave combat. Eventually he died, but none of the people who released and resurrected could loot him, so our guildmaster wrote a ticket and eventually we got our loot.
Another time, we had all 4 dragon orbs light up from the start, but for some reason we decided to just wipe and reset. Would have been pretty funny trying to kill him like that though.
When 2.4 came out, you could climb up on to the mast of the ship next to the flight path on Sunwell Isle and attack people without aggroing guards, and if they attacked you back, the guards would attack THEM and you'd get the credit for it. You could also enslave the infernals and park them on top of NPCs so if anyone tried to use the NPC and accidentally right clicked the infernal instead, the guards would kill them. This was particularly funny when the badge vendors first opened up and everyone was trying to use them. They had a passive AoE immolation thing too which didn't aggro the guards, so you could kill AFK people with them. Also, on the 2.4 test server, you could enslave the Brutallus that you saw when you flew over Sunwell for the bombing run, but I never tried it myself. They added a big invisible wall there and made it impossible to enslave him when 2.4 went live though.
There's always a whole bunch of graphical glitches as well if you play in Linux with an ATi card. The minimap goes blank when you're indoors and occasionally the icons show up as completely random textures, especially in loot windows.
Here's a few other funny things that aren't necessarily glitches.
There was a pretty fun bug when patch 3.0 came out as well. They messed up a new talent that gave your demon a buff. The Voidwalker one was supposed to increase the threat from Torment 200%, but instead it increased it by 200 times. You could solo quite a lot of stuff with it without having to be careful of pulling aggro. I think somebody even tanked a few Black Temple bosses with it.
I quit WoW over a year ago though, so I don't have any recent bugs.