There are examples I can recall, but not are all obvious at first unless you knew prior:
- Channing Tatum in War of the Worlds (starring Tom Cruise, Dakota Fanning) as “boy at church” (uncredited)
- Bruce Willis in The Verdict (starring Paul Newman, James Mason) as "courtroom observer" (uncredited)
- Sylvester Stallone in Klute (starring Jane Fonda, Donald Sutherland) as “disco dancer” (uncredited)
- Megan Fox in Bad Boys II (starring Martin Lawrence, Will Smith) as “dancer in club” (uncredited)
- Vin Diesel in Awakenings (starring Robin Williams, Robert De Niro) as “hospital porter” (uncredited)
- Brad Pitt in No Way Out (starring Kevin Costner, Gene Hackman) as “party guest” (uncredited)
- Michael Biehn in Grease (starring John Travolta, Oliva Newton-John) as "school athlete" (uncredited)
- Gerard Butler in Tomorrow Never Dies (starring Pierce Brosnan, Michelle Yeoh) as “crewman” (uncredited)
- Tobin Bell in Manhattan (starring Woody Allen, Diane Keaton) as "man on street" (uncredited)
They are not mentioned on the end credits, so you wouldn't really know unless you either know which scene they appear due to their screen time being that brief or knew how old they were upon the films release anticipating they're younger back then. I mean, have you seen movies where an actor you recognize began their career as a extra before they achieved stardom?