Factual error: The electrical power grid fails, depicted by a series of shots of city blackouts and equipment shutdowns. When the power goes out, the lights of an oil refinery go out as well as the flare. In actuality the exact opposite happens to a flare when powered equipment such as compressors and cooling devices fail simultaneously. The flare is not an electrically powered device and is designed to stay lit during power outages. All excess pressure is immediately vented to the flare stack within a second, causing an enormous flame and smoke cloud that will be visible for several miles.
Continuity mistake: When the scientists are walking through the fog towards the sphere shortly after it has landed in Central Park, we see them walk through many lines of trees before reaching the sphere. Camera cuts to a different angle, when the fog has all cleared and we briefly see behind the scientists and it only shows one line of small trees quite a bit away, not the lines of trees they were walking through before.
Factual error: As Klaatu is walking down the train platform at Newark Penn Station, in the background you can see signs that say "VIA". VIA is the national railway of Canada and does not have a presence in New Jersey. This scene was filmed in Canada, as evidenced by the signs on the platform and the 1950s style streamlined passenger cars on the track. Neither Amtrak nor NJ Transit uses those type of cars in their trains.
Revealing mistake: When the guys are seen piloting the UAV's, they are using Saitek X52 Pro joysticks. These are commercially available controllers for Microsoft Flight Simulator. These were obviously used as props. Even if the army did use these for some reason, they weren't turned on in the movie (the buttons glow green when turned on).
Factual error: US UAVs twice attack the aliens with Sidewinder missiles. Sidewinder missiles aren't a normal payload on UAVs, aren't meant to attack ground targets, and, being IR guided, wouldn't be able to lock on ground targets in either event. What is more, their warheads are comparatively tiny, further limiting their usefulness in this task. The most correct missiles to use would be Hellfire missiles.
Factual error: MQ-9 reapers are prop driven, not jet powered UAVs, and are operated by USAF not the Army.
Factual error: Crashing a reaper into a tank would not make the tank explode. In fact it would barely scratch it. The technology required to pierce tank armor is very complex. Simply crashing a large UAV into it will not work.
Continuity mistake: When Helen and Jacob walk out from under the bridge, at the very end of the movie, we see footsteps being made in the metal "sand". When the camera pans to a shot from behind, the footsteps made in the sand are gone.
Continuity mistake: When Dr. Benson is being escorted from her home, the headlights of both lead motorcycles are on. A few moments later, while on the highway, the headlamp on the right bike (looking at the screen) is out. It is back on again in the next scene. Rather odd, since on all modern motorcycles, the headlamp comes on with the ignition.
Revealing mistake: Toward the end when they are being chased by the military through the city, there is a quick shot of a military radio in one of the HUM-Vs. There are 2 antenna ports on the left side of the radio (I used to work on them), neither was hooked up to an antenna, yet the radio was transmitting and receiving.
Factual error: Near the end of the movie, the protagonists speed through a check point at perhaps 50 mph. The soldiers begin a pursuit and in moments are right behind the black SUV, which has not slowed down. Although a military Humvee can travel at 70mph, it accelerates from 0-30 in slightly over 9 seconds, and the soldiers could not have possibly caught up so quickly.
Other mistake: The US Army and civilians are shown being transported around in several CH-46s. CH-46s are US Marine Corps transports. The US Army only operates the much larger CH-47.