When Kirk's double is scratched by Janice Rand, watch where the scratches appear. On his left cheek, right? Now watch those scratches, espeically when he puts on the makeup to cover them up. You'll see that sometimes they're on the left, and sometimes on the right! The effect is best seen in the final confrontation on the bridge, where the 'evil' Kirk has the scratches on his right cheek, and the 'good' Kirk has them on the left!