This morning riding on level ground at about 18 - 20 mph, I had my rear derailleur hanger snap in half. The result was a sudden and violent stop with the derailleur being snapped into two parts, the rear hanger being completely sheared from the bike, the chain chewing up the chain stay, the rear wheel coming out from under me, and several spokes broken.
For the life of me, I can't figure out what caused the failure. I wasn't accelerating or pushing especially hard. All I heard was a sharp snap, and half a second later, my rear wheel comes completely undone. My heart rate went straight into the red zone as I unclipped and was actually able to dismount prior to hitting the pavement. I really don't know how I avoided injury, but found myself standing on the side of the road holding a bike with only one wheel on it. The rear wheel was laying off to the side of the road, and the rear derailleur was in two pieces laying about five or six yards back.
A rider 30 or 40 yards behind me stopped and said he had never seen anything like it. He said he heard the snap and then saw me riding on just the front wheel for what seemed like 10 yards before getting off. When he saw the damage, he too was at a complete loss to understand what had just happened.
So, I'm off to the LBS this evening to see if they can explain what may have happened, and if any of this is covered under warranty. What a morning!