Factual error: The British Major is wearing a RAMC (Royal Army Medical Corps) cap badge (incorrectly mounted). Really, not a British Army Corps known for their sniping skills.
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: 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: The Royal Canadian Mounted Police officer who phones Jesperson to tell him he has not been accepted is a colonel. In common with most Canadian police forces, the RCMP uses British-style civilian police ranks (inspector, superintendent, etc), not military-style ranks as the American police do.
Factual error: After the introduction, there's a scene supposedly happening near Lausanne on the Lake Geneva, but in fact it's altogether the wrong canton, Ticino and most likely Laggio Majore, a couple of hundred kilometers away. (00:06:00)