I think we have a mismatch of definitions. By “correct usage”, I mean it’s grammatically correct, but not necessarily that the exception does actually “prove the rule”. Anything that fits the sentence but doesn’t actually provide a rule-proving exception is what I’m referring to as “incorrect usage”.
Although come to think of it, I don’t think any exception can prove a rule by itself, actually. The only time it would work is if the entity enforcing the rule explicitly calls something out as an exception-- in which case, the thing proving the rule is that they acknowledged the rule by explicitly calling something an exception.