I have a pretty deep network of prefabs. Occassionally, I would get into a state in which every save (to make the asterisk on the prefab disappear) will cause these errors. At which point I must hunt down which GameObject is the problem by try deleting things and press save again, from top level, eliminating choices one by one and going in 1 level at a time. But in the end I would ended up at the TMP submesh object (that is considerably deep and used by multiple prefabs, probably equal to the amount of Asserting failed shown). Do you have any idea why this could happen? I notice this occur when I do something related to changing fonts, but not consistent enough for a bug report. I guess the outer prefab is also trying to create a new submesh while I change the font around in deeper level, and something conflicted? On a related note, do you think that this kind of generated object should have HideFlags.HideAndDontSave ? Maybe it could prevent this kind of problem in the first place.