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.
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.
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.
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.