College physics professor gives a weekly lecture. Toward the end of the first day, a student in the first row points out an elementary mistake in one of the equations. Prof congratulates the student, announces that every day there will be an error in the lecture. The midterm and final exams will consist of a list of lecture dates, and the only way to pass a given question is to point out the error in the corresponding day's lecture.

Prof gets into progressively more complex subjects. Everybody takes good notes. After the final, that student from the front row visits the prof's office, apologetically explains that nobody could figure out the mistake in the last lecture. Prof says "That's alright, I can't either."