Nastiest thing I've seen is Tony Saunders, pitcher for the Devil Rays, who broke his humerus bone in his arm simply by throwing a pitch in a regular-season game.
http://sportsillustrated.cnn.com/video/sports/2000/08/26/saunders_4059_6076.500.mov.html
You can't make it out very well in the video (Quicktime, BTW), but I saw it up close on Outside the Lines - just about made me puke.
It was said he was a heavy steroid user and put on more muscle than his tendons/bones could handle