The origin of the phrase is the aphorism that "all rules have an exception". So, when someone claims something is a rule and you find an exception, that's just

The origin of the phrase is the aphorism that "all rules have an exception". So, when someone claims something is a rule and you find an exception, that's just the exception that proves it's a real rule. It's a joke, essentially, based on the common-sense meaning of the word "rule" (which is much less strict than the mathematical word "rule").