You wouldn't be able to close the bolt on a .308 in a .300 chamber. Worst that could happen is to need a rod to knock out the cartridge. My only miscue like that was going hunting with my .25-06, but accidentally taking .30-06 ammo with me. I tried to chamber the cartridge and it jammed in the rifling. No leaning rod handy, so I got to sit in the truck while my brother hunted until dark. The source of the problem was that back them I formed .25-06 cases from .30-06. After that, I started forming them from .270s so I would have different head stamps. The .270 cases were given to me by friends and family who did not load.
