Ran across a pet peeve and decided to remind everyone that "the exception proves the rule" is a legal defense, for instances such as you get a ticket in a parking spot under a sign that says "No parking except on Sundays", but it's Sunday, so the exception proves you had a right to be there, ticket dropped. It does not mean if the sky is green one day it proves the sky is blue. It is not a rule of logic.
@Jeber That completely different context and interpretation made me google it, and, "Henry Watson Fowler's Modern English Usage identifies five ways in which the phrase has been used," cool, I may find four new soap boxes for my pet peeve.