9. Matt Damon - EuroTrip
Matt Damon may have earned a rep for himself as the King of the Unexpected Cameos nowadays, but back in 2004 that wasn't really the case, such that his appearance in sex comedy EuroTrip came as a complete surprise.
Damon shows up out of nowhere as Donny, the frontman of a band playing at a high school party, who promptly announces through the medium of punk-rock song that he's been having sex with protagonist Scotty's (Scott Mechlowicz) girlfriend Fiona (Kristin Kreuk).
Watching a skin-headed Damon rip through the indecently catchy tune "Scotty Doesn't Know" while making out with Fiona on-stage is jarring in the best possible way.
According to Damon, the cameo came about because the actor was old college friends with EuroTrip's writers, and also happened to be nearby shooting Terry Gilliam's The Brothers Grimm.
Better still, because Damon was wearing a wig for Gilliam's film, he agreed to have his head legit shaved for the EuroTrip cameo.