During one of the performances, Else was shooting some stagehands playing checkers. In one corner of the room was a television set. Playing on the television set, while the stagehands played checkers and the opera company played Wagner, was The Simpsons. As Else judged it, this touch of cartoon helped capture the flavor of what was special about the scene.
Years later, when he finally got funding to complete the film, Else attempted to clear the rights for those few seconds of The Simpsons. For of course, those few seconds are copyrighted; and of course, to use copyrighted material you need the permission of the copyright owner, unless "fair use" or some other privilege applies
LIBREria.org's Free Culture by Lawrence Lessig