Angra Mainyu is the cause of much derpage for me.

My favourite is trying to step through him for Double Vision and then backpedal back into position while he's animation locked... Except I used a skill while I was behind him, causing me to auto-face and backpedal in the opposite direction to what I was expecting. Cue the boss doing a 180 and killing half the DPS...

Oh yeah, and facing him towards the hourglass when I'm not being careful enough with positioning. Bye bye melee...