On Friday May 20th Blizzard deployed hotfixes to fix certain mobs with special effects like Grotesques and others that would occasionally bug out and could not be killed.
[blue]
The following hotfix is now live:
General
- Fixed an issue where monsters with special death effects, such as Grotesques, could occasionally not be killed (5/20)
[source]http://us.battle.net/d3/en/forum/topic/20743556157[/source][/blue]
I’ve not encountered this but I have seen animated gifs on the diablo subreddit showing annoying situations especially with bugged unseen Grotesques attacking players. That had to be very dangerous in HC.