If you are channeling Warpath, then use Vengance while still holding down the Warpath key, it would sometimes lock you into channeling Warpath. On a funny note, once you run out of mana, you stop using warpath, but your character still keeps spinning until you use another skill.
EDIT: It turns out you can also trigger this bug by simply tapping the Warpath button instead of holding it down.
Log:
Player.log (285.2 KB)