I'm one of those students who doesn't call teachers on their mistakes because half the time, the teacher is right and the only reason I think they're not is because I'm too ignorant to know better, and in all but a few cases when the teacher is wrong and I -know- the teacher is wrong, it's something piddly that doesn't matter, and I don't want to embarrass them by being a snot and stopping the whole lecture to correct them, and the rest of the time, I hate them, they hate me, and I don't want to give them an excuse to fail me.

So if you were my teacher, you'd be convinced that I wasn't paying attention, when in reality I was trying to be polite ;)