Calculus, freshman year. It was a lecture class with 300 students in the hall.
I was in one of those 300-student calculus classes too. The instructor was like Professor Kingsfield in "The Paper Chase", if you ever saw that movie or TV show - extremely well respected, very serious, and the type nobody ever questioned. He'd walk in a side door at the front of the room, give the lecture, and exit the same way. The room was always very quiet when he was present. I was always good at math, but I wasn't prepared for what happened one day shortly before finals week. The professor walked in as usual, but before he started the lecture he looked up at us and said "Where is _____ _____?" using my first and last name. I stood up and he looked at me but said nothing more. I sat down and he started his lecture. I had never spoken to him personally, and I was embarrassed to have 300 people looking at me without warning. I should have been braver and asked him why he wanted to know which student I was, but I didn't. My friends told me it probably meant I was the best student in the class, but I never learned that for sure. Maybe he just thought my name was interesting.
Lest you think I'm posting in the wrong thread, I'll answer the question at hand. My hardest class was a physics class in fluid dynamics. I had a professor that wasn't very good, a T.A. that wasn't very good, I missed one week of classes that put me behind in my work, and somehow Daniel Bernoulli and I never agreed on principles.
The "C" I got in that class was the lowest grade I ever got. Other than one homework problem, which I described
here a year ago, I did not enjoy that class.