Only recurring dream I have is I'm back in college and it's the end of a semester, suddenly I realize there's some math class I haven't been attending and I have just a few days to cram everything I've missed in time for the final.
In my case, there's some real-life precedent for it. I enrolled in one course in undergrad, and dropped it right away my first week or so for some reason I don't recall. Toward the end of the semester, before finals, I found I was still enrolled, but had never set foot in the classroom or anything.
I was sort of panicked, but went and talked to the professor, and he was fine with it. He basically verified that I never did anything with the class and it was somehow deleted from my record completely.
I still don't know why it wasn't dropped in the first week, although this was before electronic registration systems.
They underlying theme is probably low self-confidence.