heh. urban legend?
well, heres one from "my campus" - ahem, I'll no it's a lie if others have heard it before.
supposedly on one of my many campuses..a math or physics prof arrived on the day of the final exam and told the students that he had a horrible weekend before he wrote the exam. he warned the students that he was extremely pissed when he prepared the exam paper, and he took out all his anger onto the paper

he apparently made his exam really difficult.
anyway, he offered the students a chance to take a 'C' in the class if they leave now else they would have to risk getting an 'F', so half of the class leaves.
he reminds the rest of his weekend, and offers a final chance to take a 'B' another half (quarter) leave.
the people that were left behind all got A's. heh myth has it that he was too lazy to make up an exam to begin with, and he figured the students knew what their chances in the class were anyhoo