Factual error: The three panzers that ambushed the Hellcats are supposed to be Mk. III's with the 50mm gun, but they only had 5 bogie wheels, instead of 6, and no driver or radio operator's hatches.
Factual error: When Joseph Gordon-Levitt is playing poker, they are using Morgan style silver dollars. The stacks of dollars are treated as $100 dollars. But a Morgan dollar is 2.4 mm thick, a hundred dollars would be 9 .44 inches high and the stacks of coins were about half of that. (00:11:00 - 00:12:00)
Factual error: Near the end of the helicopter scene Jenko pulls the pin of a grenade with his teeth before he throws it. It is not possible to pull a grenade pin with your teeth as it requires more force than that.
Factual error: The alien drops the video camera out of the UFO from outer space and it falls to earth landing on the ground. This very far drop would certainly have busted it to pieces, if not ignited it on reentry, but when the guy picks it up off the ground, it appears only the screen is cracked.
Factual error: A scene shows a 1980's Ford Crown Victoria with New Jersey license plates. They start with QQ which means it's registered as a historic vehicle which means it has to be at least 25 years old. The film is set in 1999.