In "The Immunity Syndrome," when a lurch from the giant amoeba creature sends the ship reeling and Scotty gets knocked to the floor of the engine room, there's a really big and noticeable dirt/dust smudge on his trouser leg when Kirk helps him to his feet. Nitpicker Phil Farrand had some good laughs at this when he wrote his 1994 guide to TOS, commenting that he expected the flagship of the Federation to be much cleaner and better taken-care-of than this.
