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It can happen any time there is a mix shift in the underlying quantity of the subgroups. It's just that random changes in quantities are not likely to be studied or reported. It's easy to generate manually though.

In order for it to count as Simpsons paradox I think there would need to be a confounding variable. It's certainly possible for it to appear spuriously, and for something to look like a confounder when it isn't, but there would need to be some type of subgroup.

The classic example of admissions does not have a missing confounding variable. It's a case of aggregation at the wrong level.