.308 out of an '06...
At my club, I once found not one but THREE blown-out .308 cases, apparently fired in a .30-'06.
Yes, the cases resemble a straight-wall, except for a very slight hint of the '06's shoulder at the extreme front end of the blown-out .308 case.
Never heard of anyone's rifle going Ka-Boom, no word of an injury at the club, so apparently the--ahem--intelligent human being who tried this, got away with it.
I gave the cases to Hunter Safety instructors to use as examples of what NOT to do.
Doing it once out of confusion I could see, although it'd have to include some bad ammunition and gun handling practices, but then to do it twice more???
Just glad I wasn't there at the time.