Had a run where the healer had popped a Rage pomander and ran into a room and somehow caused a centaur and a marolith to constantly collide with one another and both become invulnerable. We couldn't attack them, but if whoever had aggro got in between the two, they would get attacked by the two mobs. Thankfully we had the passage unlocked, but it took us going "maybe we should just wipe" before we figured a way around them so we could use it.

It was entertaining, but here's hoping it is something that is getting fixed, even though it hasn't happened in any runs since that one.