It would have developed between 28,000 PSI and 33,000 PSI in a normal chamber.

If I had found a case like that that I had reloaded there would have been a full stop to all shooting of that batch of ammo until I determined why that happened. And every round in the batch would have been taken apart and inspected.

My ammo gets inspected three times during my reloading process. I keep that case on my reloading bench as a reminder that even the commercial guys make mistakes and I must never miss something that dangerous.