Scary story with a funny reason from the track last Thursday:

During the C points race one of the rider's in the group dropped his chain. It dropped on the cones, so it didn't lock up. He also stayed calm and handled his bike well. So dispite the very scary incident everything was just fine. The next day the rider took his bike to the shop to find out that the two of the bolts holding his steel dropouts to the frame had fallen out. So while pushing around 80rpm, he moved the dropout and his chain fell off.