Several years' experience as an English teacher has taught me literally innumerable things. But experience doesn't necessarily do you good. It can have terrible side-effects. One of them exerted its impact on me today. I believe this is true of other teachers.
The side-effect can be labled "difficulty addiction." When you are experienced as a teacher, and have solved many difficult questions in entrance examinations, it is possible that you have already got addicted to the pleasure you get when you have solved such a knotted quesion that no students seem equal to it. This addiction will often cause you to deal with so difficult questions in your class, letting you pay no attention to whether they are approriate in terms of the students' abilities. Needless to say, too hard problems won't do any good to them, denying them opportunities to make progress.