This is, as I recall, literally the origin of the term?
Thou shalt not kill, except all the times thou shalt.
My memory is this came from Cicero and was about a place excluding women by rule, as they pointed to the exception of a woman that was allowed.
https://en.wikipedia.org/wiki/Exception_that_proves_the_rule seems to support both interpretations, and at least shows I got the speaker right. Not seeing that my specifics are good, though. I would not be shocked to know that I am wrong there.