Over the years there have been a lot of bugs, some game breaking and some minor annoyances.
Many were just balance problems or mechanics breaking such as vanish not working and the infamous ignite muching that still exists today.
Some were altogether more... interesting.
Like corrupted blood, where a boss disease could be spread outside the instance and affect other players, causing widespread chaos as people ran around trying to kill as many other people as possible with this debuff.
Personally I think the best was back in TBC when the model for Fel Reavers was somehow replaced by the model of a normal sized bear.
Your screen would start shaking and you would look around frantically, but as the fel reaver was now just a bear you wouldn't be able to see it until too late.
Then a bear would one shot you. It was beautiful.
Which one did you think was your favourite?