I've had teachers who were more distracting from the course material than they were actually instructive -- if they made attendance mandatory, I'd have dropped them. There was one teacher (fantasy lit.) who made participation mandatory. Each class, we'd get assigned a book to start to read, and the next class, we had to start discussing it. The first one was C.S.Lewis, The Lion The Witch and The Wardrobe. The next class, we each had to say one thing about how what we read so far had brought "joy" to us.
She never got to me, and I dropped the class after that. I mean, the only thing that would have taught me would be how to say things that someone of authority wants to hear -- which, admittedly, I'm still not very good at.
And then there's was this one comp-sci professor who always wore short-sleeved shirts, even in the middle of winter, claiming it was a "mind over matter" thing.
Epic tangents. That's what I learned (and, admittedly, just went off on one).