There are examples I’ve seen but not are all obvious unless you knew beforehand:

  • 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)
  • John Krasinski in State and Main (starring William H. Macy) as “man carrying golf clubs”
  • Sylvester Stallone in Klute (starring Jane Fonda) as “disco dancer”
  • Jeff Goldblum in Death Wish (starring Charles Bronson) as “freak #1”

I mean, have you seen a movie where an actor you recognize began their career as a extra before they were stars?