I have never had this happen before, but I had a lidded pot do something strange.
I fire my lidded work together. That is, lid on pot. What happened was a weird kind of bubbling. But I'm just calling it that for lack of a better term , because, 1. there were no air bubbles in the clay body and well, there just too many things directing me away from air bubbles and 2 on the lid, it looked as though the clay had been pushed together to create these small mounds, bumps, in the piece. I might be able to find a picture if that would be more helpful.
Can y'all tell me what caused this?