The exception that proves the rule

 
Sometimes when something happens or exists contrary to how it is always supposed to happen or to be we hear the phrase "The exception that proves the rule
". This implies that every rule has an exception that proves it is, in every other case, correct. A strange concept and, in fact, not the one that the saying was designed to demonstrate.

In this saying the original meaning of the word 'prove' was test. So when something strange happened it was the exception that tested the rule. A far more sensible meaning.