I took a geology of the planets in our solar system course and literally didn't understand a single thing. it is beyond me why I didn't drop the class when I had the chance. only course in college i ever completely failed.
I took mineralogy and thankfully managed to drop the class before the midterm.
I started studying for the class and while I understood the written part just fine, there was also a lab portion around identifying minerals based on a bunch of different categories and I had NO idea how to do that.
A lot of money down the toilet, but at least it didn't hurt my GPA.
I also took Java during the spring one year, didn't do great but well enough to pass, then I took Java 2 after forgetting everything over the summer. Was taking 3 other classes and my grades were slipping so I had to make the class pass/fail to avoid the hit to my GPA and just stopped doing anything for the class.
I was also taking physics classes while being a class behind in the math prereqs. They said it would be fine if I was taking them concurrently, but one of the first classes I remember seeing a symbol and asking my friend "what's that?" and it was an integral, something I didn't learn until the NEXT Calculus class.
Managed to do well enough in then anyways even without the math, but my GPA was pretty rough for a while.