Originally Posted by Pete Hamer
Check where the chain broke. Was it the "factory installed" pin? If the links were starting to seperate they can snag the derailleur and wrap it up. When I worked at a Trek dealer I found about half of the connecting pins were installed incorrectly.
Other than that all I can say is stuff happens. It sucks but riding bikes can be dangerous.
I'll look at the chain. I didn't check that. Thanks for the info! I've been riding for about 20 years, and have never seen anything like this!