It wrapped itself around the rear cassette and took the rear drop out with it. Any clues as to how and why this would happen? I can only think that one of the pulleys seized or the hanger snagged on the cassette. This happened at a very slow speed up a not too steep hill in think clay/mud. I was in the largest ring at the back and only use a single on the front.

The chain and cassette are a few rides old and have survived a few hours downhill last Friday with no hint of an issue. I also have no recollection of ever hitting the mech or something getting snagged.

Hmmm…

This is the bike, although I had different wheels on at the time:

Lower limit screw not adjusted correctly and the derailleur went into the spokes.

“one of those things”

chains can randomly do this, I’ve seen it on perfectly adjusted systems. Stiff/off link, catching on something?

stick into the chain?

No damage to the spokes, no sticks and I’m confident the limit screw was fine as I’d checked it quite recently (I wouldn’t rule it out though but no way of checking now). One of those (fu***** expensive) things.