On the first monolith, the third quest of the monolith; upon killing the fourth soul chamber to initiate the boss fight my character is getting stuck.
I found (after three tries) that the bone/flesh piles you destroy in the beginning are still acting in the battlefield as invisible terrain. I was playing Rogue and got caught shifting into the invisible terrain and becoming unable to move afterwards resulting in unavoidable deaths.
On the same level, it seems that potions drop a layer below the ground (perhaps something to do with the necrotic damage that spills out) making them unavailable to pick up if they fall into that outer ring.