Actually, "the exception that proves the rule" means that a recorded exception to a rule can be used as proof of the existence of an otherwise undocumented rule

Actually, "the exception that proves the rule" means that a recorded exception to a rule can be used as proof of the existence of an otherwise undocumented rule. It dates back to medieval legal terminology. http://en.wikipedia.org/wiki/Exception_that_proves_the_rule http://en.wikipedia.org/wiki/Exception_that_proves_the_rule