This complete destruction occurred after the chain hopped off the front ring into the spider (happening quite frequently at the moment, despite my attempts to adjust the front mech). I tried changing down at the front to get the chain back on, but by then it had caught up in the spider and developed a double-loop (where the chain does two small full-circles on itself). In my lazy impatience I decided to carry on pedalling; as the knotted chain passed into the rear mech, the whole lot just exploded, and this is what the pictures show.
If you get a knotty chain, don't try to pedal it out!
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