If all were fair and good in the world, then learning would be linear. You'd learn something, and then always build right onto that. And then you'd build onto that. And you'd be able to take what you've learned in one context and apply it in other contexts.
Alas, all is not fair and good in the world, and learning is very non-linear. This can be frustrating for students and instructors alike.
For example, you can take a fairly advanced college English major, one who seems totally able to cite correctly when writing a paper, one who's been practicing citation all semester on short assignments, and give that student something different in an assignment, something just not quite what they've done before, and kablooey! s/he forgets all about citation.
And then s/he appears in my office, upset at having failed an assignment in large part because s/he didn't cite appropriately and protesting that s/he always cites usually.
I sometimes think that the hardest parts of teaching are trying to get students to carry what they've learned in one class or context into another, and conversely, not wanting to smack students upside the head when you realize that they haven't carried what they learned in one class into the new one. (The urge to smack students upside the head is especially strong when you've taught them something specifically in the earlier class because it will be helpful in the later class, and you've told them as much.)
It's especially frustrating when they give you that look and say, "but you didn't tell me I had to cite!" And then I want to say that I didn't tell them to wipe their ass when they were last in the restroom, either. But I don't. Listen, you're in an upper-level English literature class. I shouldn't have to tell you to cite your sources. It should be second nature, like wiping your rear.
This is the frustration of the day, brought to you by the letter F.