Somewhat similar to some of the posted ones, my father relays me this story about a professor he TA'd for:

The professor taught a class with a single student enrolled, who would sometimes show up late. However, the professor would always start the lecture on time regardless, and so the student would have to quickly take notes to catch up. If the professor filled up the board before the student arrived, he would simply begin overwriting the old material, and the student, once he arrived, would be in a race with the professor to copy it down before it was erased. One day, the student missed the class entirely, and the professor gave the entire lecture to an empty room.

I thought this story was apocryphal, but my dad said it's likely true, as it was fairly specific about this professor.