Attention conservation notice: the comment below is not substantive at all.

For the pedants, to say something is an exception that proves the rule is to say that when you look at the exceptions, they're so unusual that it reinforces the point that the rule is generally valid even though it isn't universally valid.

Actually, for the real pedants, to say something is an exception that proves the rule is to say that an explicit written exception to an implicit rule is evidence that the implicit rule exists, e.g., a sign that says "Parking prohibited on Sundays" implies that parking is permitted on other days. The full expression is, "The exception proves the rule in the cases not excepted."