This is twice in about 500 rounds that I have seen this happen:
Instead of popping out whole, the primer fractured and only part of it came out of the primer pocket. I thought the first instance was just a very odd fluke. But this is twice in a very small set of used brass and I don’t recall this ever happening before in the over 100K times I have removed primers.
Update: I had this happen with three more cases out of about 150. All with R-P headstamps. I bought this brass used. It would be interesting to know the history of these cases.