I started carrying a chain tool after hearing multiple horror stories about breakage/snapped derailleurs/hangers etcetera. I wasn't religious about bringing it though. Lo and behold I'm mashing up a climb on some rocky singletrack and all of a sudden my drivetrain seizes up and I hear a sickening snap. I look down and my hanger is bent and cracked, the derailleur millimeters away from my spokes, and a small stick sort of woven into the chain and derailleur.
I had one of those park tools allen wrench mini sets so I took the derailleur off and sort of slung it around the chainstay. Because I couldn't shorten the chain, however, I could not turn the bike into a single speed and actually pedal. Bummer. The good news is it was all downhill on the way back. The bad news is that I coasted the last 4 mile stretch of road home at about 5 mph during the first snowstorm of the year. Major bummage.