Mine happened on a case that was from an unfired cartridge which I pulled the bullet with a scale on a bullet puller to see how much pull weight it would take to remove the bullet. This is a necessary number to know when manufacturing or re-manufacturing 22lr ammo. Which is my primary hobby.
So it was a case with a full primer charge. It made quite a pop with the gasses escaping down the punch. It did not rupture the case but ballooned out the case head pretty nicely. haha. Good thing too because my face was on the other side of that case when running it in the derimmer.