Corrected entry: The planetary saucers in the movie are 15 miles in diameter, and are 2.5 miles in height. Assuming that 9/10 of the ships are air or lighter then air gases, and that the ships' walls are made of a metal with the density of water (this is a very generous estimate), then each ship would weigh 100 billion tons! No matter what source of flight used, these ships would create a pressure of 30 atmospheres on anything below it. Most buildings would crumble under 5 atmospheres, and our bodies would flatten like pancakes long before that.
Corrected entry: When Captain Steven "Steve" Hiller and David Levinson are getting ready to fly out on the alien ship, the camera shows Hiller the launchpad for the missile that they are to use on the mothership. Hiller says, "It's just like the AMRAAM launch pad on a stealth." AMRAAM stands for Advanced Medium Range Air to Air Missile. Both the F-117 "Stealth Fighter" and the B-2 "Stealth Bomber" only carry air to ground weapons. Neither stealth is equipped to carry AMRAAMs, and therefore wouldn't have AMRAAM launchpads.
Corrected entry: The entire scene where Will Smith and his girlfriend first see the spaceship is pretty ridiculous. First, Will goes out to get the paper, but instead of taking it back in the house, he opens it up and starts reading it - in the yard. Then Jasmine comes out (conveniently neglecting to notice the spaceship that's directly in her line of vision) and offers him a cup of coffee - in the yard. Were they planning to spend the rest of the morning standing in the front yard? (00:25:20)
Corrected entry: In the scene where Will Smith first notices the spacecraft, he is absolutely astounded by the sheer size of it. Yet his girlfriend, bringing him coffee, doesn't seem to notice it until she walks up to him and looks at his face to see what he is looking at. She didn't notice the 15 mile long UFO? It's not a really a mistake, just really crappy writing and acting.
Corrected entry: The Secretary of Defense turns towards the President and gravely announces "We can expect the destruction of all major cities in 72 hours". Wow, I'm impressed. Either the US or Russia could accomplish that in less than half an hour. Incredibly advanced technology ain't what it used to be...
Corrected entry: When in the area 51 base, a screen in the background shows a night vision view of a plane landing. Why is a Panavia Tornado fighter bomber landing in an American secret base, when they are flown in England, Germany and Italy?