Friday The 13th Pt 2 - Wrong Cheek
Around 18 minutes into the movie, the counselors are all arriving at the camp. In one scene, Terri, a girl counselor is walking in the woods with her dog. Another counselor, Scott, is hiding in the bushes with a slingshot. He shoots her in the butt with a pebble. In the close-up of her butt, you can see that the pebble hits her in the left butt cheek, but she grabs her right one instead.