Everyone makes mistakes, and actors are no exception. Movie Mistakes cover the best known ones:
(Star Wars): Star Wars When the stormtroopers break into the control room, watch very carefully and you will be able to see a storm trooper nearly render himself unconscious by smacking his head off a door frame. This is now so notorious that on the new DVD there's an audible "bump" when he hits it.
(Commando) The yellow Porsche is totally wrecked on the left side, until Arnie drives it away, and it's fine