Director Steven Spielberg accidentally stepped into the frame of one of his first movies, a made-for-television production called Duel. When the film was released on DVD, Spielberg left the mistake in to remind him of the imprecise nature of film making. Other famous movie mistakes include the miraculously self-recovering hubcaps in Bullitt, the clear shadow of a modern movie camera in Stagecoach, and the door-challenged Storm Trooper in the original Star Wars.
Why do such movie mistakes occur? Quite often the answer lies in the high pressure world of commercial film making. Directors generally work under tight budget and time restraints, and re-shooting scenes can be a logistical nightmare. If an occasional movie slip-up manages to sneak into the final cut of a film, it may be best to leave it in rather than arrange for an expensive re-shoot or other post-production fix.
Some movie mistakes are caused by the standard practice of shooting out of sequence. Very few movies are filmed in a linear progression according to the script. The producers may only have access to a particular location for a short amount of time, so the director and actors must shoot all of the scenes based at that location at one time, regardless of where the scenes fit in the overall narrative. This leads to continuity errors, such as having a dirty costume suddenly appear clean again. Shooting out of sequence can also lead to inconsistencies in an actor's appearance or use of certain props.
Continuity problems in movies can also be triggered by the long process of lighting, arranging and filming a set. During a dinner scene, for example, the actors may first perform the "master shot," a wide shot of the entire dinner table with all actors eating and speaking. For close-ups of individual actors or smaller groupings, the entire set may be completely re-arranged and re-lit. It is nearly impossible to keep track of every single movement of every single actor during several days of filming, so the resulting scene in the film could have any number of mistakes, as silverware changes position or food disappears and reappears at random.
Other movie mistakes are the result of factual errors or anachronisms. If a movie is set in a particular time period, such as the 1920s, it is up to the set designers and others to make sure all props and backgrounds are historically accurate. In the film O Brother Where Art Thou?, for instance, one scene includes an audience singing along to a performance of the song "You Are My Sunshine." In reality, the song was not even released until several years after the presumed date of the movie. The song's lyrics worked well artistically, but it would be considered a movie slip-up by film buffs.
Sometimes cast or crew members create movie mistakes by stepping into the shot or allowing a piece of equipment to appear. Boom microphones are especially difficult to wrangle without dipping them into the frame, so many modern cameras have built-in safety zones which prevent such accidental intrusions from microphones or crew members. This is the basic error Steven Spielberg committed during the shooting of Duel, stepping past the established safety zone while filming a climactic scene between actor Dennis Weaver and an unseen truck driver.
Considering how collaborative the film making process can be, it is actually amazing there aren't even more major movie mistakes. Databases such as the Internet Movie Data Base(IMDB) include movie goofs and continuity problems, largely contributed by observant movie buffs. Some movie mistakes are not actually mistakes, however, but inside jokes or deliberate incongruities created for effect by directors or producers. A few hardcore film buffs will analyze every frame of a favorite movie in search of technical or artistic bloopers, many of which would be considered too arcane or obscure for average moviegoers.