Hard to pick just one... there are several that stick out in my memory.
First class I ever had was Mechanics... first test I ever had too. I got every single answer correct and finished it 15 minutes... and was given a 47 on it. The professor accused me of not showing the work I did on a calculator, but I didn't even use a calculator. It was all super simple math, like "What force is needed to accelerate 10kg at 10m/s²?" F=ma, so the answer is 100N. Yeah, I lost points for not "showing what I did on a calculator" there.
Another annoying class was Calculus, wherein I had a visiting professor from berlin, that was so single-minded that he'd lapse into german while explaining an equation, and it'd take half the class yelling at him before he realized he needed to speak english again. Then he'd basically act like we were all idiots for not being multilingual polymaths and would often refuse to explain things a second time, even if no one in the class understood it yet. He'd simply say we were already behind schedule, and if weren't paying attention the first time, that was our own fault. No way could it be his poor explanation to blame.
There were a lot of other cases, but for the most part, those were later in college, I learned to see the signs early, so I'd drop a class in the first week and switch to another one when I sensed doom. As for the stupidity of my fellow students, well... that actually came as a boon to me more often then not. As the saying goes, "When you and a dwarf are being chased by an hungry dragon, you don't have to outrun the dragon, you just have to outrun the dwarf." So yeah, the slower the dumbest students were, the easier the grading curve would be, and the less work I'd have to do. The obvious exception to that is for group/team projects, but I was always really good at picking hard workers for whatever group I was in.
