Calculus III... I think I could have got it, but it's tough material and I had a terrible professor, so I didn't really learn any Calculus III. I passed and did much better in my final math course, a one semester differential equations/linear algebra hybrid course.
Also, my first semester I took one class of "Honors" calculus which was a proof based differential+integral Calc course. I dropped that really fast in favor of Calc II. So I also hit a wall at proof based calculus. Most of the people who actually took the class didn't get it either and resorted to memorizing all the proofs needed to derive calculus.
