You shipped the chain? Is it geared or fixed?
Geared. I’ve no idea what happened other than the cranks spun as if there was suddenly no resistance. Chain was off and inboard once I picked the bike up. Chainring teeth are very pointy and short.
Even quite worn chainrings don't normally throw a chain though but it could happen.
You looked at the derailler adjustment? Chainring bent or loose? BB movement?
@PhilDAS started
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You shipped the chain? Is it geared or fixed?