Back in those days, it was required that in order for a student to receive credit for a particular course, a card (listing of his/her courses) had to be signed by the instructor. It was, at the time, policy that students attend their courses. But depending on the size of the class, it was often quite possible to receive credit, even after not attending the class regularly.
Not so, with this physics professor, if he didn't recognize you, you would have to repeat the course (& attend!). On one occasion, a student handed his card to be signed. The professor looked at the name, then at the student, and said, "I've never seen you in my class," and handed back the card.
Now being a science student, he naturally thought quickly, and proceeded to the end of the line. When he was at the front again, he handed his card to the professor. The professor looked at the name, then at the student, and said, "You look familiar" and signed the card.