Students are amazing creatures. Over the past year I've had some interesting encounters with mine--including a PhD candidate who originally thought the class was worthless because his English was good enough, but stayed in class because he found it useful (and his English ability was impressive). But, for every student that impresses me, there will be some that disappoint.
I only failed two students last year--one didn't show up to the last few classes or the final exam and the other just didn't do the work. The latter is in my class again this year. Thus far, he has slept during most of the class and neglected to hand in the first assignment (second assignment is due Friday). And at the beginning of the year he asked why he failed--I can think of a few reasons.
Then there is the case of the elitist students. Three PhD students who have not done much of anything in class except talk in Chinese. One of them left class to take a phone call for half an hour. I was going to make a sweeping statement about how it's rude to speak during a teacher's lecture, but they decided to talk through that. And so, I singled them out, thus making them lose face in front of their classmates. I went back to my office to look over assignments and found the three students whose assignments are of very poor quality--they didn't follow simple instructions.
A note to inconsiderate students: If you're going to be rude, you better have the ability to impress the instructor with your work. Otherwise, you'll probably fail.