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.
The following hotfix is now live:
- Fixed an issue where monsters with special death effects, such as Grotesques, could occasionally not be killed (5/20)
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.