As a sensitive little kid, I was traumatized by the opening scene in the PG-rated James Bond movie "Live and Let Die" where a guy gets stabbed to death, and so I was under the impression that all movie ratings beyond 'G' were due to upsetting violence (as opposed to profanity, adult situations, etc).
So when "Ferris Bueller's Day Off" came out and I saw that it was rated PG-13, I wondered to myself what kind of violence the movie contained. In my mind, I pictured "Friend of Ferris" (Cameron) being like "Hey Ferris, I hear it's your day off!," and Ferris going "Yep!," and stabbing the friend.