I formed a rule; I would go to the first lecture of any series and if it taught me anything, I would go to the next, and so on. Though I rarely got past the first couple of lectures, there was one lecturer I kept going to because he was so astonishingly wrong. He lectured on Byron, whose work he misunderstood so fundamentally that I had to spend a year writing a research paper on Byron's comic verse before my accumulated bile was purged. That lecturer's woeful performance has convinced me of one thing; in the world of intellectual cut-and-thrust, a truly incompetent teacher can be of more value than a good one.
That may be true of university lecturers. Not sure us high school teachers could get away with it though.
Mind you, I often say this to my classes:
I used to say to my students, "Confusion is the most productive state of mind. Respect your confusions. Don't let me waft them away."
And am guilty of this on a daily basis:
The very best teacher is the one who really enjoys being made to look a fool by a student.
Maybe I'm not so bad after all.