Factual error: At the end when they are loading Dewey into the ambulance on the stretcher, it is standard procedure to put the patient neck brace when they receive any type of unknown injuries (stab wounds) to their back as Dewey did - it was notably absent.
Factual error: The portrayal of the firing MLRS before Denzel's unit moves off is ludicrously inadequate. The rockets launched from the MLRS resemble a space shuttle lift-off, but in the movie it was more like a sparkler thrown into the air.
Factual error: We learn that the previous sheriff, Charlie Wade, took revenge on anyone who smuggled illegal immigrants across the border in the late 1950s - we are shown the smuggling, as Eladio Cruz hides a group of Mexican men in a watermelon truck to take them across the border. However, in the 1950s, the border was not fortified the way it is now - the smuggling shown would simply not have been necessary.
Factual error: There is a major error in the scene where Geena Davis steals the tractor trailer. They show a closeup of the brake line between the truck and the trailer being cut. The problem arises when fluid starts pumping out of the line and Geena screams because she's lost the brakes. First, every tractor trailer is equipped with air brakes, so there is no fluid in the lines to leak. Second, if these air lines are severed it causes the brakes on the trailer to lock up and drag the truck to a stop.
Factual error: The geography throughout the film is all wrong. The setting is in Pennsylvania, near Pittsburgh. The driving times between there and Williamsport are extremely under-estimated. Most egregious is the river structure. The body is found in the river near Sunbury. Not possible from a site in Western PA. Basically all relationships between actual towns/cities mentioned are totally wrong.
Factual error: At the Oro Array, the breakers are turned off because the power is cut. When the alien cuts the cables to the computers, sparks fly out as if there is power.