18. George Clooney Has Guest Starred...As Stan's Dog
Once South Park started getting popular, a few celebrities offered to voice characters on the show. Trey Parker and Matt Stone thought it would be funny to book these people, but instead of having them voice anyone significant, they'd just give the celebrities super unimportant roles with no lines.
In Season 1, George Clooney guest stars in the series, and who does he play? Stan's dog, of course. All Clooney does is provide a few barks and that's it. It's virtually impossible to tell it's George Clooney even if you know ahead of time. Jay Leno also appeared in another episode...voicing Cartman's cat. Yeah, Leno came into the studio and they recorded him doing a few meows. In the episode City on the Edge of Forever, Henry Winkler shows up too, providing the growls of a monster character. Once again, there's no way to know who's doing the growling.
Those people were all in on the joke and thought it was funny, but a few celebrities weren't as amused. Jerry Seinfeld offered to be on the show, and Parker and Stone agreed, but then they informed him he'd be playing Turkey #2 in their Thanksgiving episode, Seinfeld declined the appearance.