Back in school I sometimes had to write unannounced quizzes from the material of the previous class. I suppose the point was to test if I have prepared for the class or not, but in multiple cases it was the feedback from the test that actually explained what I have previously misunderstood or didn't think fully through. That's a good thing, feedback from testing is a very valuable learning opportunity. But it is bullshit that I was punished by a bad grade for not fully understanding the previous class' material. If it was older material, for which we have already checked homework with the teacher, I would say okay, but that wasn't the case and I remember feeling betrayed. It certainly did not help me not hate studying.