Beginners accidentally clocking each other too hard without a mouth gaurd is pretty common.
I've never seen any tooth damage, but I've seen some pretty mangled lips. The lips are soft and fleshy and teeth are hard and pointy. Always ends badly.
Joe Lauzon>Jeremy Stephens>Marcus Davis>Shonie Carter>Matt Serra>George St. Pierre>Jake Shields>Dan Henderson>Fedor Emelianenko