Hey, It's That Guy! - Justin Long, the Mac Guy (from the "I'm a Mac, and I'm a PC" commercials) in his first film role as one of the Galaxy Quest fanboys.
Based On — There was a now mostly-forgotten Star Trek fanfic that had a similar plot. "Visit to a Strange Planet Revisited" was published in the 1970s in Star Trek: The New Voyages, a collection of fanfics edited by Sondra Marshak, Jacqueline Lichtenberg, and Myrna Culbreath. In the story, William Shatner, Leonard Nimoy, DeForest Kelley, and James Doohan step on the transporter set, and end up on the actualU.S.S. Enterprise in a crisis with the Klingons.