Chemistry was worse than math for me. Somehow they expected you to remember a variation of a formula from way far back, and understand that you could now use a different notation system to derive another, third formula from a new formula that you had just learned... but didn't explain that and just threw that new third formula (with entirely different units/inputs) at you and it always was a slog to go track down how it all went together because the mental concepts just didn't flow. I don't even remember the name of the textbook or the professor of the class, but I still remember those stupid blue boxes in the textbook where mental mindfuck took place.

I've tutored calculus, and probably the biggest example I've seen of this is the difference quotient. The formula is exceedingly obvious once you understand it, but it takes a lot of people some time for it to "click".