Was riding a dumpster-found mountain-bike-shaped-object yesterday, one that I had already serviced/cleaned/inspected/test ridden (hard, I might add), and heard a strange grinding coming from my rear derailleur. I looked down and saw that the chain seemed to have jammed between the plate of the derailleur cage and jockey pulley... well, me being with a friend at the time and only about two blocks from my house, I kept pedalling hoping it would jump back on. After a few strokes I heard the unmistakable sound of something grinding against spokes, yelled out and expletive to my partner and coasted to a stop at the bottom of the hill. The jockey pulley was missing, the plates misaligned, and the chain had managed to jump from the middle of the cassette to the inside, wedging itself between the "pie plate" and the largest cog. Seems the derailleur cage was what was smacking against my spokes, but the spoke guard managed to keep the chain from tangling in the rear wheel.

If I cared about keeping the bike, the pie plate saved a lot of potential damage to the spokes. Then again, if I cared about keeping the bike, I would not have picked one up with a Tourney derailleur. At least whoever at the local bike co-op gets the un-enviable job of replacing the RD won't have to worry about replacing spokes (given how often I'm there it'll probably be me in any case).