My worst learning experience took place when I was a graduate student in a physics program. The professor teaching the quantum course the fall of my first year would get lost in the middle of derivations, often erasing several lines of equations before starting over again. His apologies for these mistakes got old very quickly. Equally as bad, he did not feel like writing the equations out for each step, erasing and replacing factors, terms, and signs several times, before writing the equation again out in full. He seemed oblivious to the fact that the students had to scramble to keep up with him, because we had to write every step out in our notes in order to study them later. I had some teachers who weren’t that great before, but at least I got the impression that they were trying their best. He wasn’t a bad person, but it seemed he never prepared for class. Fortunately I had a good teacher for quantum as an undergraduate and there are several good texts on quantum mechanics at this level. We were able to convince the department to let the professor who normally taught this course to teach the spring, but the damage had been done. There was a noticeable discontinuity in the course between the two semesters.
This wasn't the end of it, unfortunately. In the fall of my second year, the faculty member in charge of the graduate program insisted that all of the second year students take an advanced course in many body quantum with this same professor. The excuse given was that learning this subject was “an important part of our education.” Since I was not specializing in solid state, nuclear physics, or anything that involved a large number of particles, I had very little need for this subject. Suspiciously, there was no insistence that everyone take a course in relativistic quantum, which would have been much more relevant to all of modern physcis . To add insult to injury the professor required us to buy his new book. He was selling at a discount because it was still just a proof. He said that if we found any errors, he would appreciate if we told him so that they could be corrected before it was published. At this point those who had a research advisor escaped by taking research credits. The rest of us were stuck. His teaching method was as mediocre as before with the same bad habits. Unfortunately there weren't many books on many body quantum at this level and his book was not that enlightening. Somehow I managed to answer the problems well enough on the exams to get a B+ in the course, but I sure that was just due to the curve. I never felt I understood what I was doing.
Fortunately the rest of my experience in this program was positive enough to offset this class, but the experience made me somewhat cynical. The story I heard was that this professor was very prolific in publishing in his field. All of the faculty in the department had a teaching requirement, though. Since the undergraduates (or their parents) were paying a lot of money, the department dumped him on the graduate students.
Looking back there were a few problems here. At the very least instructors should prepare for class. It would also be helpful if they got some training in how to teach. One specific point: every time a change is made to an equation in a derivation, it needs to be written out again in full. (This should be obvious, but apparently for some this is not.) There also were issues with the role of teaching in the department. How important was it relative to research? There were a few other instances where corners were cut on the teaching requirement with disastrous results. Ideally if scientists teach poorly and are unwilling to work at improving these skills, is there some way to put them in a research center affiliated with the university where teaching isn't a requirement?