Calculus was the worst for this: in my first class we spent literally two months dissecting the minutiae of limits and Lipschitz conditions and infinitesimals and blah blah etc, only to get the the punchline, "and you find the slope of a function by doing the obvious thing, which works in the obvious way every time you'll actually be using it in practice". I get it in a college level analysis class or something, but as an intro in high school that's just a great way to make students hate what is at its core a very simple and useful subject.