There are examples I’ve seen but not are all obvious unless you knew beforehand:
- Richard Dreyfuss in The Graduate (starring Dustin Hoffman) as “boarding house resident” (uncredited)
- John Krasinski in State and Main (starring William H. Macy) as “caddy / judge’s assistant” (uncredited)
- Bruce Willis in The Verdict (starring Paul Newman) as “courtroom observer” (uncredited)
- Channing Tatum in War of the Worlds (starring Tom Cruise) as “boy at church” (uncredited)
- Brad Pitt in No Way Out (starring Kevin Costner) as “party guest” (uncredited)
- Sylvester Stallone in Klute (starring Jane Fonda) as “disco dancer” (uncredited)
- Jeff Goldblum in Death Wish (starring Charles Bronson) as “freak #1” (uncredited)
- Vin Diesel in Awakenings (starring Robin Williams) as “hospital porter” (uncredited)
- Gerard Butler in Tomorrow Never Dies (starring Pierce Brosnan) as “crewman” (uncredited)
- Megan Fox in Bad Boys II (starring Martin Lawrence) as “dancer in club” (uncredited)
I mean, have you seen a movie where an actor you recognize began their career as a extra before they were stars?
This isn’t exactly the same thing, but Matthew Fox played a paratrooper with no lines in World War Z. This was after Lost, so he wasn’t an unknown by any stretch. He originally had a bigger part, but they changed the entire 3rd act in reshoots and his role was reduced to basically nothing.