Please state in which country your phrase tends to be used, what the phrase is, and what it should be.
Example:
In America, recently came across “back-petal”, instead of back-pedal. Also, still hearing “for all intensive purposes” instead of “for all intents and purposes”.
That’s actually a post-hoc rationalization; in the original phrase, “proves” has a meaning closer to “tests”. But, yes, people use this one all the time to justify being wrong either way.
https://en.m.wikipedia.org/wiki/Exception_that_proves_the_rule
Probably not according to Wikipedia.
and how is that post-hoc?
If I claimed I didn’t get a ticket that day because I wore my lucky socks that would be post-hoc. I don’t see how that applies here.