It was all the shortcuts, left out "intuitively obvious" steps, and sloppy use of variable names and symbols that drove me up the wall trying to learn advanced math from (imho crappy) math profs.
Let me tell you about the lecture with no less than three different Epsilons (a rounded one, a less rounded one, and something in between) used in the exposition of the proofs. That and my slight myopia kind of did me in in that class (or would have if I hadn't cribbed notes from my neighbour.)
Or, when, as a math prof, giving an example of applied math used for something like physics, why don't we just pick completely random variable names, instead, of, say, using m for mass consistently, v for velocity, etc.
I understand some kind of need to force people to think only of the form and not imbue terms with more semantics that aren't necessarily there in a particular mathematical formal system, but holy cow, could you be more obscure? Why yes I could. I will use 's' for mass on this page, and r' for mass on this next page, just to see if you are paying attention. !!!