I found a blue mob that wouldn't die.

I was clearing The Keep Depths level 1 in Act 3 on inferno. In a corner of one of the pathways in the lava room right after the waypoint, I found a blue Electrified Illusionist Nightmarish Desecrator icy quillboar pack. All went fine killing the pack until I got to the last one. It flat out would not die. It would fall to 1 hp, and then nothing. it also would not attack me or my followers.

Anyone else have this happen before?