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Ah, overproofing. You can try and save as much of it as possible by putting the loaf in a baking tin, though it will have an odd shape anyway.
Do you bake a lot?

Well not even overproofing, but instead the sourdough only killed itself before the baking even started. Odd things happen.

I don't bake as much as a serious hobbyist or a "real baker", but I've baked far more than average men ever will bake :)