Revealing mistake: In the scene at the stadium where Lisa is helping Neville escape from Mathias' people, it is extremely obvious that in the far shots, it is a stunt man (wearing a very bad grey wig) driving the motorcycle, and Charlton Heston in the close up shots.
Visible crew/equipment: Very early on in the movie, Neville is happily driving around the deserted streets. Half way through this scene the camera shows us Neville's view out the front windscreen of the car. Ahead, moving cars can be seen crossing the intersection, obviously where the streets hadn't been blocked off for the filming of the movie.
Revealing mistake: When Neville gets back to his fortress-apartment, he boots up the video surveillance system, switching on several closed-circuit monitors. The very largest screen, however, shows himself, looking toward the screen. As he turns about his image on the screen doesn't turn the correct way; it moves like a mirrored image: he turns to the right, the image turns to the left. The image ought to turn in the same direction. The image in the video screen was photographed separately (and not quite inaccurately), and added later in post-production.
Continuity mistake: In at least one scene, traffic lights are shown flashing. Where's the power coming from?
Other mistake: Early in the movie, when Neville wrecks his car he takes the gas can out of his car and starts walking down the street. As he does they switch to a wide view of the city in which you see about three city blocks. When you look at the only "through" street in the shot, there is a car making a left hand turn about three blocks down that street and just behind the car is the freeway overpass with two or three cars passing on it, before the shot ends.
Other mistake: In the opening scene where Charlton Heston drives into a Deserted Los Angeles, he drives past at least 3 people walking down the sidewalk. The first person he passes is quite visibly holding up the traffic for the shooting. Another can be seen on the left of the screen when Heston puts the tape on. And one more can be seen in the wide shot of the street. Other than this, the film makers did a great of making LA look deserted.
Continuity mistake: Near the beginning, Neville sees the sun just above the horizon, about to set, and he flees for his shelter, racing through city streets. As the camera repeatedly cuts to follow him, it's obvious that the sun is still high in the sky, nowhere near sunset. In fact, the sun's position is constantly changing from one shot to the next all throughout this film.
Revealing mistake: Watch when Dutch throws the flash grenades during the stadium scene. When seen igniting on the ground, they are positioned closely behind large boxes and other bulky debris, to hide the special effects used to create the light. It would be virtually impossible to toss the grenades any distance and then have them drop down into place so perfectly on the far side of the conveniently placed obstacles.
Factual error: In the opening scenes of a city deserted for several years, lots of smog can be seen at the base of the mountains in the distance. Smog is entirely man made and would disappear completely in a matter of days once all human activity stopped.
Deliberate mistake: During the opening shot and titles, Neville is shown driving by the same block three times, each from a different angle. The first shot is high, the second shot (00:01:50) is from the hood of the car looking at Neville, and the third (00:02:40) is ground level.
Factual error: In one scene, Neville is sitting alone in a theater watching "Woodstock". He has obviously done so many times since he has memorized the dialogue. The projector, however, appears to be running itself. Theaters at that time used dual projectors and it was necessary to change reels approximately every 15 minutes, but there is no sign that Neville does so. One might also wonder why Neville would deliberately choose to sit alone in a darkened room where the "tertiary cases" could easily trap him.
Continuity mistake: In the opening scene when Neville almost hits the armored car, he crashes his car into the curb. The shot shows the car from a side view hitting the curb, and the tire going up over the curb, still intact. Next shot shows Neville getting out to check the damage, and the tire is now flat.
Continuity mistake: When Lisa sees the family marching towards Neville's house, in front of the building on the left is a waste can and a mail box on the opposite corner. When Neville drives by the same building a short time later, there is now a dark car parked in front, two other cars parked on the sidewalk further down the street, and the mail box has moved across the street by the door.