Obviously, a seating error. My guess is the bullet was a FMJ instead of lead. Note the unusual damage to the rim of the case.

Yep, that case mouth got folded all the way over and pressed flat in seating or firing...

And now the true horror and mystery begins to unfold:

And gets even more unbelievable:

And makes you wonder how a primer this badly canted in the case could have ever fired:

Anybody wants to expound some theories of how this happened, it would make for an interesting discussion.