Continuity mistake: In the interior shot, when Jasmine tries to explain why she doesn't want Steven to leave she pulls open the white curtains, to view the alien ship through the window (the window closest to the door), and as Steven walks out the front door she stands by that open window. Then when it cuts to the exterior shot, in an instant that window's brown venetian blinds are now down and the white curtains are drawn closed, as Steven walks down the lawn.
Continuity mistake: When Steven is dragging the alien in his parachute, the sweat stain on his tank top changes shape and gets smaller, by the time he speaks with Russell a few moments later.
Continuity mistake: In the old Plymouth on the way to DC, when David comments, "Can't this thing go any faster?" the lock on his door is down as the shot fades in on him, but when it cuts to his close-ups the lock is up, unlocked. (00:33:20)
Continuity mistake: The missile used in the mother ship had a 30 second count down. From time of launch to explosion was one minute thirty-six seconds.
Continuity mistake: In the kitchen, when Steven is packing up before leaving for El Toro, the blue dolphin figurine faces the TV and is about six inches away. In the following shots it faces the opposite way and is about three feet away. (00:28:50)
Continuity mistake: When the White House is being evacuated, in the shot from behind as Connie rushes to the chopper, at her side she carries a briefcase in her left hand, but in the next shot facing her nothing is in her left hand (viewer's right). (00:42:50)
Continuity mistake: After Major Mitchell is asked get a gun to shoot the Coke can, in shots facing David, the Major's body faces toward the alien attacker, but in shots facing the alien attacker Major Mitchell's body is turned the opposite way.
Continuity mistake: When the White House is being evacuated, as Connie runs down the stairs in the shot facing her, she is speaking with a woman to her right, but in the next shot from behind the woman has vanished. (00:42:30)
Continuity mistake: When Steven is about to head back to El Toro, he tells Jasmine that she and Dylan can come stay with him at the base. Then when they kiss, Steve's head leans to his right - in shadow, then left - with lots of sunlight, and back to right - in shadow again, between shots. (00:30:30)
Continuity mistake: Aboard the AWAC, after the pilot sees the fireball emerging from the clouds, there is a wideshot of the interior with the cockpit in the background. The pilot is not wearing the dome-shaped helmet, note the distinct outline of his hair, whereas in the close-ups he is wearing it. (00:16:10)
Continuity mistake: In the Oval Office, when General Grey says, "They're tracking another one off the California coastline," as Connie rushes in, the lamp on the table beside the couch is gone. Then in the overhead shot of the Oval Office the lamp is back on the table. (00:14:45 - 00:15:50)
Continuity mistake: On the AWAC, in the close-up of the pilots' hands at the console, they are wearing gloves that reach up to their wrists. When the fireball shatters the windshield, their gloves are much longer now - closer to their elbows. (00:16:10)
Continuity mistake: After David makes a mess in the Area 51 alien attacker's hangar, when he is sitting on the ground facing Julius, the liquor bottle label changes directions in his hand. (01:35:25)
Continuity mistake: After going to the bathroom, when Steven turns on the TV in the kitchen, in the TV's close-up before the program comes on, the kitchen's reflection is visible. In the reflection something is lying on the stove (beside the fridge) and a chair is in front of the corner door, but when it cuts to Steven opening the fridge door, they're gone.
Continuity mistake: When the spaceships begin arriving, it is obviously morning in Los Angeles. Will Smith and his girlfriend are still in bed, and later on Will goes out to get the morning paper, etc. Jeff Goldblum has just said that the countdown will be over in roughly 7 hours, so how is it night in LA when the spaceship finally attacks?
Continuity mistake: During one of the big building explosions we see a police officer on a horse. He turns on the horse, and then the camera cuts to the next shot and we see him turning on his horse again. (00:23:45)
Continuity mistake: When Steven Hiller goes out to get the paper, the paper has a string around it to keep it closed, yet he opens it up as if it was only folded. (00:25:15)
Continuity mistake: In the scene close to the beginning of the film when Steven Hiller is talking to his friend in the locker room, the ring accidentally falls out the locker, and they both look behind them, yet in the following shot they are facing forwards. (00:36:05)
Continuity mistake: In the middle of the film when the President is in Area 51 looking at the alien aircraft, there is a shot from the back and you can see that the General is not standing really close to the doctor, in the following shot from the front he is standing side by side with the doctor. (01:14:10)
Continuity mistake: When Dr. Okun and David are inside of the alien ship, and Dr. Okun shows him the scrolling green lines on the console he says that they don't know what they are. David says it's the same thing as their countdown signal and they always move together. Yet when Steven is flying their ship through the mothership, the lines go horizontal, spread apart, and even turn blue in some parts. (01:21:00)
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.