During the scene in the original King Kong where Kong has Ann just after he fought the snake in the cave, he takes her up onto a cliff. He sets her down and walks to the cave's edge and bangs his chest. He then goes back to her. As he looks down on her, if you play it frame-by-frame, you will see a tool left in my the animators. It is a metal rod to mark the progress of the animation. It is in just for a frame but can be seen next to Kong. It appears bigger than him!