Leaving for a ride this .morning dropped the chain into the small ring as I was getting going and the drive bound up tight. Stopped and looked down and the chain back wrapped over itself. I couldn't figure out how to undo it even with the rear wheel off without pulling the crank. So I just grabbed a different bike and fixed it later when I got back. I saw someone else do this a couple years ago, bu no idea what happens to cause it. What a PIA! Fortunately I was only a couple hundred.feet from the house when it happened.