I think everyone who shoots long enough experiances this at least once in there lifetime.
my experiance came from shooting 308 out of a 30 06 barrel in a browning machine gun. The cases looked like straight wall 45/70 brass when i was done with it.
the barrels/calibers are easy to change on the BMG and, israeli 1919 bolts will shoot either 30 06, 308, or 8mm so its very easy to forget which barrel you have installed to the gun.
I now make certain to check the barrel before i shoot each time cause it is easy to forget.
you are not alone