Here's my bad teacher story:

When I was 13 or 14, my physical science teacher was talking to the class about space probes with trajectories that take them outside the solar system. He said that such probes get faster and faster as they go. Thinking he either had misspoken or was intentionally being wrong to see who would catch his error, I corrected him. To my surprise, he said he had not misspoken and that he was correct. We argued about it a bit then he told me to write down a defense of my position.

Later that day, kids came up to me and said, "Why are you arguing with Mr. S? You know he's right!".

I wrote a weak attempt at a defense of the law of inertia (using a reductio ad absurdum argument if I remember correctly). When I gave it to him the next day, he praised it and conceded the argument -- but only privately. He never admitted he was wrong in front of my classmates.