Have I told you learning calculus is hard? At least, when you try to use formal approach (i.e., the approach used by mathematicians). Physicists, and I suppose engineers too, tends to assume a certain function is well behaved and manipulate it right away (derive, integrate, etc). Mathematicians seems to worry a lot about what seems to me who have physics background, petty things. Well, admittedly it is fundamental, like existence theorems. Still it is annoying.
But I suppose learning the fundamentals helps to think more carefully and less sloppily. I hope so.