I rented out a room in my house to someone, for a few months, and after they left I found this in the bathroom. It was soaking in water (terribly messy renter), and kind of freaked me out. I flushed it down the toilet ASAP.
Goes out of focus when clicked on. Anyway, your main problem is you may now have rot and mold in the wood.
It is likely to be Peziza domiciliana, but it is hard to tell for sure from the photo. But this would be a normal mushroom to find in a damp bathroom. -frog
