I have had this issue happen twice so far. While in the defiled crypts i came across an elite pack of Ghostly Murderer's with the illusionist affix. When they split apart one of the illusions became immune to all damage. The creature had no life bar and when you attacked it no message came up but he will follow you all throughout the dungeon poking you.

It's not a big deal but since its a bug i figured i would post this. Thx