McLean’s Optical Illusions: The Desert – optical illusion disc which is spun displaying the illusion of motion of a ball with a wedge-shaped piece missing passing through a hoop and of a monkey swinging on branches of a tree and a zebra jumping through an opening between two trees in a circle at the outer edge of the disc.
This famous stunt in the movie was actually built around what went wrong with the original stunt. Keaton intended to leap from one building onto the roof of another building, but he fell short, smashing into the brick wall and falling into a net off-screen. He was injured badly enough to be laid up for three days. But when he saw the film (his camera operators were instructed to always keep filming, no matter what happened), he not only kept the mishap, he built on it, adding the fall through three awnings, the loose downspout that propels him into the firehouse, and the slide down the fire pole. (The Three Ages - 1923)