In a well-known tort decision, Judge Learned Hand observes that while legal standards almost always coincide with customary industry standards, strictly speaking custom never controls. This Essay examines the implications of this apparent paradox, concluding that courts must have final say in order to prevent doctrinal feedback loops—situations in which legal doctrine influences customary behavior which, in turn, influences doctrine, which in turn influences custom, and so on. Were feedback loops allowed to develop unchecked by judicial review and intervention, they would lead to unfair and inefficient overinvestments or underinvestments in care. The Essay describes the approach courts should adopt in determining whether, in given instances, these feedback loops present a problem.