I've notice this happen quite a few times now, mostly in spider rooms, where there are a lot of bones laying around the floor. I'll be running around looting the spider sacks or whatnot and kick a bone and take damage! It's the craziest thing, it's only like a hp or 2, barely even noticeable, but my character grunts like he's just been svcker punched in the gut. It's pretty funny when it happens. I can only liken it to stubbing your toe on a bone or something lol.