Continuity mistake: Close to the beginning when David is outside the White House and puts the device on top of his car, in one shot you can see him put the device on the top and put his hand down, and in the following shot he's putting his hand down again. (00:40:10)
Continuity mistake: When Russell boards the F/A-18 and when he is airborne - such as when he salutes Major Mitchell (who is back at command center), he wears his Hawaiian style short-sleeve shirt and is gloveless. After he says, "Eagle 20, Fox 2," in the close-up as he presses the launch button (resulting in launch failure), the arm shown wears both a green jumpsuit and gloves, neither of which Russell actually wears.
Visible crew/equipment: When the President stands by "the big tamale" he asks Dr. Okun if he can tell them anything useful about them. When Okun rushes up the ramp as he passes Nimzicki, the actor's mark is visible on the ramp, directly in front of the Secretary of Defense. (Only visible on fullscreen DVD.)
Continuity mistake: Close to the middle of the film when Steven Hiller and his wingman Jimmy are running from the smaller alien aircrafts, Jimmy says, "I think I'll try something", and in this shot we can see Jimmy's fighter jet go up, and in the following shot, we see it go up again. (01:00:50)
Continuity mistake: In the scene at the end of the movie where David and the President are making amends, as you view the President from David's POV (face on) his head is tilted to his right shoulder, however, as you view the President (from behind) as he is looking at David his head is tilted to his left shoulder. This is noticed each time the perspectives change. (02:14:50)
Continuity mistake: Although the 'line of sight' scene has been mentioned, in one particular shot from behind Jeff Goldblum, he has his right hand out with the piece of paper in it, but when the shot switches to in front, he has his hands on his hips. (00:45:10)
Continuity mistake: Given the direction that the spaceship is traveling as it passes over the Mall in Washington, D.C., the White House should be completely in shadow by the time the Capitol is covered. This is not the case. Also, the shadow should spread over the White House from the left to the right, but instead it goes from the front to the back. (00:22:45)
Visible crew/equipment: A couple of shots after Jasmine grabs Dylan from the car, the rears of a red car and white car are shown tossed up from a front shot. Next shot from behind, the same two cars are shown tossed up again where we can see the flat belt system between the four tires and the mechanical arm at work doing the flipping. (00:53:40)
Continuity mistake: At the end of the movie, all the falling debris and the accompanying firetrails change color all at once.
Revealing mistake: When Will Smith and the other jets are flying into the ship and are firing their missiles there is a cloud in the background that doesn't move. You can tell it's a background set piece. (00:58:20)
Visible crew/equipment: After General Grey shows the nuclear missile and launch mechanism to Steven and David, when Connie asks, "Why you?", in the next shot just as a woman in a white lab coat passes David, his yellow actor's mark is visible on the floor at his feet. (Only visible on fullscreen DVD.)
Continuity mistake: When the Casse kids are in the motor home and the guy drives up to complain about Russell's cropdusting, Miguel stands at the doorway. In this shot he leaves his hand on the door frame, in the following shot it's by his side, then back on the door frame. (00:10:15)
Continuity mistake: When the alien aircraft settles over New York, there is a taxi cab driver that stops to look, and the truck behind actually hits it twice. Also, there is a parked yellow school bus which the front half of the taxi passes in the wideshot, but in the close-up, when the driver gets out, the taxi is actually much farther back, the bus is gone, just take note of the gate in the background. (00:22:25)
Continuity mistake: At the end of the scene, when Connie tells David that she has never stopped loving him she is standing in a doorway. The close-up shot shows her arms are folded across her chest, but when it cuts to a longer shot her hands are on her hips. (01:26:10)
Continuity mistake: When Captain Hiller ejects from the F/A-18 he is clearly strapped into his seat, but when he lands the seat is nowhere to be seen - it's not on the ground and he's not sitting on it. Then shortly afterward it instantly appears near him. (01:05:35)
Factual error: Area 51 is located in southern Nevada (the movie acknowledges this), but the security gate that Steve Hiller enters through is shown to be in the Bonneville Salt Flats, which are located hundreds of miles away in northwestern Utah.
Revealing mistake: When Marty is stuck in traffic in his car, trying to get hold of his therapist, there is an overhead shot just before this one showing traffic with a few taxis. This traffic shot is flipped - note the steering wheels on the right side of the cars. This particular shot is shown again, but not flipped, just after the Empire State Building blows up. (00:44:00 - 00:47:10)
Continuity mistake: When David shows up at his father's house, Julius opens the door pointing a gun at his son. David grabs hold of the barrel and in the shots facing Julius, David lets go, but in the shots facing David the barrel is still pointing up which David is still holding firmly.
Continuity mistake: At the decimated MCAS El Toro, when the First Lady asks Jasmine what she does for a living, the First Lady holds the bowl out with her left hand in the shot facing Jasmine, but in her right hand in the shots facing facing her. The direction of the bowl's handle also changes.
Continuity mistake: As we hear the biplane's engine closing in on Miguel Casse, as he rides the motorbike down the road, in the overhead descending shot his shadow is to his right, but next shot as his crop-dusting father passes over him the shadow is to his left.
Answer: Remember that the aliens had to interface with our satellite computer code first...David simply "reverse engineered" the code to create the virus. When it was uploaded, they didn't have enough time to combat it.