a particular case that is so unusual that it is evidence of the validity of the rule that
This phrase comes from the Latin legal maxim exceptio probat regulum in casibus non exceptis - exception proves the rule in the j cases not excepted. This in fact meant that the recognition of something as an exception proved the existence of a rule. But the idiom is popularly used or understood to mean a person or thing that does not conform to the general rule affecting others of that class.
1998 - Spectator - The success of The Full Monty in the United States is an exception which proves the rule. On such lucky breaks, industries and economies are not built.