Other mistake: As the camera pans in on Woltz's mansion right before the horse's head scene, his bedroom is on the second floor. However, the windows in his bedroom show a courtyard in the background. The horse's head scene was filmed in a dining-room-turned-bedroom at a Guggenheim mansion on Long Island. The Guggenheim dining room is located on the first floor, as shown in a February 2022 episode of Antiques Roadshow.
Other mistake: After Sonny is shot on the causeway, there are a very large number of bullet exit holes visible on the sides of the tollbooths, which could only have been made if the gunmen shot through the walls (not the windows) of the buildings from the inside. This is common in movies. In real life, bullet entry holes in wood appear as small black dots, not the more dramatic-looking splintered gashes made on the other side of the wood as the bullet exits. (01:52:40)