I've broken my clavicle twice racing. The first time the cause was a rut in the road that I hit as I pulled off the front, the second was when a guy in front of me panicked as he closed in on the riders in front at the bottom of a little incline, hit a wheel (I think) and went down right in front of me - in the direction I'd chosen to go around and move up. Not sure what the lesson learned is there, except that riding really fast, really close - in the half-light of dawn especially - is risky. Also, strong, muscular shoulders wouldn't be such a bad thing for a bike racer to have.