I (thankfully) never have, though I've shot plenty of Big Bores and magnums. Maybe that's the secret... I'm defensive when I shoot them!
But my second son has twice... from the same rifle, same load, same circumstances. On 2 moose hunts with his Rem 700 in 338 WM. The 3 - 9 X 40 Leupold got him! First time above the eye and second time on the nose... blood both times, but he was unaware of it until I told him!
But the worst I've witnessed was quite a few years ago at a military range in Nova Scotia just outside Halifax. About 100 hunters were sighting in rifles and all from prone. The guy on my right was well built in his 30's, shooting a handloaded lever-action Winchester in 45-70, with a BIG scope... from PRONE! (I did mention that didn't I?).
At the command to "commence firing" all sorts of pandemonium broke loose and I just happened to notice that the big guy with the big gun and the big scope wasn't shooting anymore... in fact he was out cold with what looked like a large dose of red liquid oozing from his forehead! Then came the command: "cease fire"!!!
It was funny... but not!
Bob
www.bigbores.ca 
