Sudden leap in difficulty – Nearly everyone noted that at some point in their learning they stumbled upon a sudden leap in difficulty that crippled their ability to grasp the follow-on concepts
I have hit this in every subject I've studied where I wasn't really motivated to learn the material. The reason this happens is that you reach a point where suddenly all the basic concepts start supporting the follow-ons. If you never really learned the basics, you will be lost. For example this happened to me with foreign languages. I had no real interest in learning French, but a foreign language was required in school. The first year was easy: basic vocabulary, stock phrases, and simple rules of grammar. The second year was not too bad either. But I never really learned anything... I would cram before exams and get decent scores, then promptly forget the material. Third year, when you actually had to put it all together... I was sunk.
If you're studying something, and can't motivate yourself to really learn the basics, you can only go so far. If you're studying computer science because you heard the jobs pay well, but you don't really care about learning the first and second material, you will hit that "leap of difficulty" at some point.