I still remember like twenty years ago undergrad probability theory a professor posed some question to the class and even though this prof was normally very thorough with being helpful and walking through answers with students, this one guy answered so wildly off-the-mark the prof paused a bit and then just said "no" and moved on.

We were doing final exam review for earlier semester material and the question was about the probability of randomly drawing some hand of cards, something like a hand of five cards with exactly three jacks. Guy answered very confidently "it's 1 minus the null set". I remember this because I immediately asked the kid next to me what was said and just heard the same thing repeated.

So many things wrong. A "null set" is a concept from measure theory, which was not used in this second-year-undergrad course. Since using "the" here implies there's just one, he almost certainly meant the empty set. That's whatever. But we're not in a set theory class, 1 is a number, not a set, so we're not in a context where it makes any sense to subtract sets from numbers. But if we just push all of that aside and say okay fine, represent 1 as a set however you want and subtract the empty set, taking any set A and subtracting the empty set just gives A back, meaning he's given an extremely roundabout way of saying the probability is 1, a 100% chance of randomly drawing that specific hand of cards.

Situation where it's would be one thing if we're early on and he'll discover he's in over his head, but right before the final is such a wild time to sound fully confident in an answer that wrong.

Moral of the story: sometimes having that much confidence behind an awful understanding will give bystanders enough secondhand embarrassment that they'll still think of you from time to time twenty years later.