An exception to a rule, which generally proves it.
How Did We Miss This One?, Up for Grabs, Needs More Examples, Needs a Better Description. Alt Title: Exception That Proves The Rule. An exception that proves a rule is one that proves the rule's existence, by being an exception to it. For instance, a sign saying "parking on Saturdays only" proves that there is a rule against parking there any other time, by making Saturdays an exception to that rule.
I'm trying to decide whether this should be just a Stock Phrase or an actual trope. I'm leaning towards it being a trope, but I would like a consensus on it first.