And they said it couldn't be done...

What you are seeing is a very nice second-degree burn I got while mountain biking. While other injuries are more common -- cuts, bruises, broken collarbones, separated shoulders, what have you -- I think it takes a special kind of skill to burn yourself on a disk brake rotor. If you look closely you can even see the curve of the rotor.

At least I had good company:
Aaron cut his ankle while riding the railroad ties at the bottom of Chaparral.
Stefan blew out his rear brake line in a nifty half-endo at the bottom of Castle.
Andrew jacked up his rear hub (probably a broken pall) while riding up The Bitch.
Jason broke his chain while just riding.

Ah, Joaquin Miller Park, how we love thee...