One thing they don't warn you about in graduate school -- unless some places have changed their "teaching preparation" classes to be somewhat more useful -- is that, every once in a while, you'll get a student who is, shall we politely say, "problematic". This is the student that takes up 80% of the time you spend interacting with students that semester, and in a negative way.
I've probably seen a few more of these students than the average, because I've allowed my course to be offered through the Harvard extension school. There have definitely been many cases there of students who just enter the class insufficiently prepared, and most of them quickly drop the class. But occasionally there's one who misunderstands and thinks it's our fault (mine and the TAs) that they're failing a class that they may not have had the necessary background for to begin with. (I've recently had to deal with such a student, which brought up this line of thinking.)
For sheer annoyance value, though, my most problematic student was a Harvard student. He or she (let's use "he" from hereon) got a warning from me partway through the semester because he failed to turn in an assignment. I told him he had done fine on the assignments he had turned in, but if he didn't turn in one or more future assignments, his grade would suffer, and he could even fail the class. He said he'd understood.
After the midterm, he did not turn in another problem set. Which would be fine, except that he then made a rather large issue out of failing the class. He insisted on knowing the exact formula I used to assign grades, going over every question on the midterm and final with me, and so on. In short, he refused to take responsibility for the outcome, which is the hallmark of a problematic student.
I'm curious if other teachers have had similar experiences, and what advice they might have in dealing with such students. (My advice -- catch these students early, and document by e-mail what they have been told regarding their performance! And try to spend more time with more positive students.)