I have had many students transfer to me from other piano teachers over the years. It's always the same story - they can't play anything well. Why? They've been pushed too hard, too fast.
See, the average piano teacher was her own teacher's star student. And the average piano teacher takes a star student and pushes. That is what was modeled for her, so that's what she does. The thing is, what about the other twenty students who aren't going to go on and be piano teachers? They are being pushed just for the sake of being pushed.
I take things a step further, and argue that even the students who show initial talent should not be pushed. Why should they be? They are already learning faster than the average. Give every student appropriate, incremental challenges and encourage their progress. They all have the potential to learn to play music well.
What is the main way that teachers push their students? It's not by being demanding in terms of technique or artistry. It's by giving them repertoire they are not ready for.
Inexperienced teachers do not have the ability to see tiny advances in a student's skills and understanding, or the tiny holes in a student's skills and understanding. These teachers leap forward in the level of difficulty of the pieces they assign without realizing it. It's kind of like those who complain that type is too small instead of acknowledging that they need glasses. And when a student's momentum falters, it's easy to blame the student for lack of effort instead of seeing that the situation was created by the teacher. The effect of this can turn a thriving student into a quitter in just a few weeks.
The thing to do as a teacher, if you're prone to this common mistake, is to err on the side of giving material that is too easy. If you do this, make it up in volume: give several easier pieces for the student to play. This builds fluency in reading music, builds confidence, and, of course, gives the student more music to enjoy. Resist the temptation to "see what this kid can do." If you sequence the student's material correctly, you'll have years to find out.