For me, this cliff came with discrete mathematics, where the appearance of proofs, induction, set theory, and so on left me completely foundering for some time.

For me, this cliff came with discrete mathematics, where the appearance of proofs, induction, set theory, and so on left me completely foundering for some time. They were all topics that I had literally never touched before except in the most brief, informal ways, and none of them seemed to have any relation to material I'd previously learned. None of it really made any sense to me, and in retrospect, the only reason I even managed a passing grade in the course was because I'd been taking a statistics course at the same time that covered many overlapping topics (sets and relations, discrete probability, etc) in a "use these things to calculate this other thing" way that actually meshed with my previous education. Proofs were the real killer out of all of it. I still today have barely any idea how to manage a formal proof or what elements are involved in one, because at the time trying to figure out what the hell was going on with this notation and formatting and abbreviations everywhere that I'd never seen before and trying to make any sense of the weird self-referencing logic of it was unpleasant enough that I just fumbled through the bare minimum to get a passing grade whenever they were involved.