In 1973, Hanna-Barbera contacted Hunter S. Thompson to guest star on Scooby Doo. For obvious reasons, the ensuing episode was simply too much for Saturday Morning kid’s TV, and it never aired, remaining sealed in H-B’s Burbank vaults.
Update: This article about Thompson’s death and its immediate aftermath would be a morbidly funny parody as well, except that it’s real, sadly.