arkansascyclocross.com broke his collarbone exactly two weeks ago at cross practice. So I can no longer say that I've never heard of anyone breaking a collarbone doing cross.
We were doing hot laps at Burns Park and were ready to pack it in, when Ryan says, C'mon let's do one more! Always be very careful when you hear those words. Especially if you are (1) tired and (2) cranky.
I took it way too fast off of the pavement onto a slight grassy downhill. I'm not sure what made my front wheel suddenly turn left, might have been a rock, root, or sand, but the end result was me flying over the handlebars and landing on my left shoulder. A (long, crazy) visit to the ER confirmed the broken bone.
Last Friday I had surgery to match the two ends with a metal plate so it heals straight. I seem to be recovering fine, each day it feels a little bit better with a little more range of motion, but I have to keep it in a sling for the rest of the month.
Moral of the story? Be careful! Any time you go downhill on a cross bike, stay loose and relaxed and keep your weight back. I've decided to put bar-top brake levers back on my cross bike for exactly that reason. [If they are good enough for Adam Craig and Katie Compton, they are good enough for me.] Even without cheater levers, you still want to keep your butt behind the saddle and ride those bumpy speedy sections conservatively; there's far more time to make up on other sections of the course.
Ride safely and have a great cross season!