I’m pretty sure I mentioned this in the past, but I found a good example, and the idea is worth repeating. Besides, the guy who made the mistake (and he might have made it on purpose. He is, after all, literate.) is a worthwhile daily read. First, here’s the quote:
So, ladies and gentlemen, it gives me great pleasure to show you what is actually not the exception that proves the rule, because in this brilliant animated piece, Rina Piccolo not only does none of those things I object to, but she makes me laugh aloud several times and generally delivers not only a bravura performance on its own but a model of how to avoid undermining good cartooning in a cross-media interpretation:
The source is Mike Peterson’s Comic_strip_of_the_day.com for February 19, 2014. Mike and I are alike in that we both like to use comics to illustrate whatever point we want to make, though I generally limit myself to matters of writing, and his topics fare far farther. The mistake is the expression “the exception that proves the rule.”
To prove in the expression “proving a rule” is an old usage meaning to test, not show. You might remember this usage from the book of Malachi 3:10, where God says “Prove me now herewith…” about tithing. (This, by the way is the only place in the Bible where God invites us to test Him.) You can find a few other examples of prove meaning to test in the King James Version, but that passage is the most famous.
So making an exception tests whether something is actually a rule, or merely a suggestion. Break the rule, and if you get into trouble for it, it’s a rule. If not, then it’s not.
My advice: If you’re going to make an allusion to an old idiom, use it correctly. Otherwise, use plain English.