My second Calculus class in college kicked my ass. I made good grades on all the homework and regular tests, but the final looked like some completely different form of math. I did not know it was worth 50% of our grade for the class.
Family law. I totally knew the material, but I didn't see the last page of the exam until it was time to put down our pens and hand in our blue books.
Classes I should have done the worst in: Nonmonotonic Reasoning - I read all the stuff, went to class, participated in the discussions, but I never was quite sure what we were talking about. Fortunately, no one else seemed to be either, so my grade wasn't too bad. Constitutional Law - As if the course does not already lend itself to being one of law school's most boring, having a professor that thinks he has figured it all out in a way that deviates from the casebook and most other material on the subject is a recipe for disaster. Again I was saved by group confoundment.