In the dungeon Halls of Agony in Act 1 just before the butcher you will encounter dark cultists. If they stand in a circle and channels they will transform into Unholy Thralls, they can be annoying to kill on inferno.
If you run in the middle of them and use an fear attack. "other cc might work" they will not transform into unholy thralls anymore and can easily be killed.
In other words. when they channel the transform interupt them. they will not cast the transform again
It bugs them for recasting the spell