This is to be filed under "lessons learned" and "stuff happens". Pemberton, New Jersey:
I was at the beginning of my fifth day of touring from Delaware to my home in central New Jersey. I was about a mile away from my campsite in the morning when I heard (and felt) a tremendous crunch from my bike. The drivetrain was pretty much frozen.
The chain was hanging in a loose loop, not quite touching the ground, and the derailer was hanging with it. It appeared it had been ripped off the bike. I got off and walked the remaining half-mile to the Dunkin' Donuts I was heading towards.
Busted derailer by neilfein, on Flickr
It's hard to make out in this picture, but my derailer snapped clean in two.
Ouch. I'd noticed on the way there that the rear wheel was now rubbing the brakes, obviously out of alignment, and probably severely out of true. I imagine a shock like this can't be good for the rear wheel. After some web searching (and a cup of coffee), I found that no nearby bike shops where I could try to get this fixed. I called my wife; she was happy to pick me up, and I settled in for a wait. (If she hadn't been able, I would have asked my friend for help, or rented a car.)
"I've seen this before", said the REI mechanic a week later. "You probably got something caught in your drivetrain, something like a stick or a rock, and it jammed things up." I told him about having ridden through the Cape May peninsula, where the air is filled with salt and the roads have sand blown on them; and also about having camped on a farm the night before, with paths of dirt and straw and pine needles.
Camp Pine-Needles by neilfein, on Flickr
Packed up and ready to leave the campsite; the bonfire from the previous night is behind the bike. Little did I know I wouldn't get far.
The final bill was just over $100 to replace the derailer and true the rear wheel. (I had them do a full tuneup and cleaning while the bike was there.)
I asked the mechanic how to avoid this in the future, and he told me that the trick is to learn to listen to your drivetrain. If you feel anything sticking or catching, stop and take a look. Pick things out of your drivetrain if needed. Even small objects can hurt. (I'd been squirting some lube on the cogs from time to time, and that can also help. It may have even bought me time until this happened.) Of course, this is an extreme case of what can go wrong!