The Simpsons creator Matt Groening has issued a warning for celebrities who turn down parts on the show - they risk being ridiculed in future episodes. The hit TV show has had a glittering list of celebrities appear in cameo roles, including Bob Hope, Kirk Douglas, Elizabeth Taylor, George Harrison, Ringo Starr, Sir Paul MCCartney, Sir Mick Jagger and Keith Richards. But Groening admits he still gets annoyed when a star turns down a part. He says, "We were told the investors in Planet Hollywood - Arnold Schwarzenegger, Bruce Willis, Sylvester Stallone, whoever - were willing to do the show if we mentioned the restaurant. "We wrote a script for them, but it turned out that some publicist made the thing up. We got our vengeance: We slammed Planet Hollywood on the show."