An APS synopsis describes a failure mode in fast-slow dynamical systems: reducing the fast variables can miss a narrow “singular funnel” in phase space and therefore predict the wrong long-run behavior. The result is technical, but the intellectual use is broad. When most trajectories behave regularly, model validation naturally concentrates on the broad basin. A thin channel can still dominate rare transitions—the very transitions a risk model is often built to catch. It is a compact argument for examining geometry before celebrating a lower-dimensional fit. American Physical Society, September 28