Misunderstood Saying

I came across a mention on a TV show the other day of the saying "The exception that proves the rule," and one of the characters said that they never quite understood that saying.

This is one of those old cliches that doesn't make sense today because of changes in word meaning. Originally, going way back, the word "proves" meant "tests." So the saying really means "The exception that tests the rule."

In other words, if you have an extreme or outstanding instance o something, it "tests" whether the rule is, in fact, accurate.