So I accidentally picked up a sign in the library, and then I put it back down. Except it didn't appear on the floor, and it was removed from my inventory, so it might be just gone and/or in need of a restoration. I reproduced it in essence as an alt, picking up a lamppost outside and then dropping it. I'm going to avoid touching anything in the meantime...

So yeah. Pick something up, and then drop it. That's the repro.

It also seems like some crafting and item dropping of other things doesn't update the inventory instantly, requiring closing it and opening it again. I don't know if that's related, but it could be.