I am interested in finding "real-world" examples of when variables might exhibit marginal independence but are conditionally dependent given some other variable. It seems to me that the converse (marginally dependent but conditionally independent) is more common (e.g. autoregressive processes, confounding variables in general).
Are there any canonical examples of this phenomenon that I am not aware of? My searches turned up many theoretical treatments of this idea, as well as numerical examples of simple distributions where this property holds, but no motivating examples from science/engineering/business/etc.