Visible crew/equipment: When Russell lands his crop-dusting plane and Miguel rides up alongside on his motorbike. The equipment used to help stop the motorbike, at the right position for the close-up, is visible on the ground - but it's gone in the wideshot. (Only visible on fullscreen DVD.)
Continuity mistake: When the B2 releases the nuke it banks away to the left, however the next radar shot shows it flying away to the right.
Continuity mistake: After David looks in the fridge for ice, Connie appears at the doorway, and when he makes a toast to the end of the world, the bottle to his right turns between shots, note the label. (01:33:10)
Continuity mistake: Aboard Air Force One, when David feels nauseous there is a telephone between his seat and his father's, whose cord gets twisted between shots, though it hasn't been touched.
Continuity mistake: When David and Julius are playing chess at the park in Brooklyn, the slow decomposing styrofoam coffee cup, David's beeper, radio, removed chess pieces (not in play), etc., change position between shots, particularly in relation to the chess board.
Continuity mistake: When Julius and David pull up beside the White House gate, David places the device on the Plymouth's hood to triangulate Connie's position inside. The device's position on the car's hood differs between the close-up and wideshot - note the car door.
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.