Factual error: Iceland sure looks a lot like the California desert.
Factual error: When the military are going to attack the flying saucers in Washington D.C., there's some shots where you can see trucks and airplanes in a deserted region. Washington is hundreds of kilometers away from the desert. (00:18:30 - 00:21:10)
Factual error: The USS Sawfish is supposedly a nuclear powered submarine, but the shots of her entering Williamstown show diesel exhaust coming from her. (Apparently they had to substitute an Australian diesel-electric submarine).