I have never had it happen to me but saw it happen once in a .577 2 3/4. It seemed to recoil a bit more but the shooter said it was nothing as he wiped the blood from his nose.

Another friend doubled his 475#2 Holland from the bench. His thumb hit him just below the eye and blackend it. It could have been the rifle.

Both times it was a slip like 500Nitro says.

I rigged up a Halibut hook on some surgical tubing to cure the problem. One end of the tubing is wrapped around the barrels and the hook is set in the crotch. People are very concious of controlling the rifle with this training aid. It seems to have more of a mental than physical application.