Trey Parker makes an actual cameo in Team America: World Police (2004).
"But Angie," you shout. "It's puppets! He couldn't possibly appear in the film!" Well sir, this be the truth. "But where, oh where doth Trey appear?" you ask.
Where else would this comic genius insert himself into his film? Into a puddle of puke (aka soup and beer mixture).
There's Trey, laying in the vom during the vertical shot. He's wearing a wig and the legs are fake to make him look like the puppet counterpart, but it's him fo sho.