In the sequence where we learn that Andy is missing from his prison cell, the warden is seen walking down the catwalk past the cells wearing a dark blue necktie with yellow diagonal stripes. As he’s walking, the knot of the warden’s tie is dark blue. But when he interrogates the guards and Red in Andy’s cell, the knot in his tie is yellow. Then when he rips the poster off the wall and peers into the tunnel, the knot in his tie is dark blue again.