For me, it was North American Geography. One trimester I had a seriously heavy load of anthropology classes and labs (my major), and I needed some way to get a few credits that wouldn't require much study. I found a 3-credit class in American Geography in the catalog, and figured I'd found the perfect solution. After all, that was a subject I'd had in elementary school. How hard could it be?
It turned out to be a class for serious geography majors, and was anything but easy. Given the load of classes and labs for my major, I didn't have the time or energy to devote to geography. The class had only two grades: midterm and final. By the time I failed the midterm, I knew I was in deep trouble. It took a long time for my GPA to recover from that class.