I am doing multiFAILs all the time

1 ) Forget about buffing in instances

2 ) Using the wrong totems

3 ) Starting a boss fight with my healer almost OOM... that's my favorite

4 ) Making outdoors group quests and forgetting to put the tank bar on my tank and heal bar on my healer, so my tank ends two shoted and the rest of the group running for their lifes.

5 ) In Coldara near Nexxus I was ganking people one day I was bored and I managed to pull to the ground one of these lvl 80 elite drakes that fly over your head. I guess it had something to do with my 40 yards flame shock and using alt tab to target everything that had red name. Since the two rogues I was about to gank vanished I decided at least to take down the drake thinking the rogues just ran away.
The drake fight took about 7 minutes or so... my group was lvl 71 by that time. At the end of the battle all my toons were OOM and almost dead... Then suddenly out of nowhere appeared two /cheer /lol emotes and the two rogues I was about to gank popped and anihilated my almost dead group.

6 ) Try to make UK with red gear tank... forgot to repair his gear before entering the instance. That would be not a big deal if I didn't realize it until I reached the second boss and saw the melee guy doing lots of damage on my tank... By that time I had really spammy overheal macro, so I could have reached that boss with a nude holy priest as tank if needed.

7 ) One day I was having fun with the target dumies in IF and saw a dwarf watching at me. When I finished my macros tuning I just made a show in front of that dwarf making figures with chain heal and dropping totems. Then another player passed by my side and said "What are you doing? Entertaining the guards?" Then I realized that the dwarf was a city guard.