Last time I rode, it was shifting fine. This morning -- well, that happened. The bike has never been crashed. The clamp-bolt was torqued to spec. That was a 10-speed Campy Chorus derailleur; it had seen about 5000 miles of service. Just seems like a very unlikely place for it to break; if it had been a manufacturing defect of some sort, I wouldn't have though it would take 5000 miles to show up.