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An interesting possibility - would the bike remain rideable after that? I rode mine the rest of the way into work OK, it's stuck on the smaller cog. If I try to shift to the larger cog it doesn't shift and is a bit noisy, as if it is trying to shift on.
I have just had a look. The circlip seems to be there, no idea what the ping was. Everything seems intact. I think the issue is that the derailleur is / was being impeded by crap. I can't believe how bogging the bike is around the derailleur area.
Lost the circlip holding the cogs on?