Patrick Dempsey
McDreamy wasn't even credited in his first-ever role. Before starring in the '80s rom-com Can't Buy Me Love (1987), Patrick Dempsey appeared in The Stuff (1985), a satirical sci-fi horror movie in which he played "Underground Stuff Buyer #2." The character was so minimal that there are no existing stills of Dempsey's feature in the film.
Charlize Theron
If you blink, you might miss Charlize Theron in her first film role: 1995's Children of the Corn III: Urban Harvest. Theron was cast in an uncredited role as "Eli's Follower." Surprisingly, the movie wasn't as bad as critics had anticipated. TV Guide wrote that the story "actually improved over time to the point where this third installment is a creditable if far-fetched chiller."