oh man, statistical mechanics (& thermodynamics) is the only class that I ever busted my ass for and still failed. That was brutal. They later broke it apart into two separate classes, but it was too late for me - I'd already switched from physics to computer engineering. Computer-related stuff, even the hardware down to the chemistry and analog electronics, came much easier.

I thought I understood the stuff theoretically, but problems with ordinary differential equations compounded the difficulties with the practice problems.