Factual error: Towards the end when the cavalry uncovers the American flag, it has 50 stars.
Factual error: When Cary Grant is attacked by the crop duster, he is supposed to be between Chicago and Indianapolis. Unfortunately, northwest Indiana looks nothing like this. The scene in the movie has miles and miles of treeless landscape. Even in the flattest farmlands of Indiana, there are many visible groves of trees.
Factual error: The galley during the battle scene is rowed by slaves. Romans during this time period didn't use slaves to row their galleys. Rowers had to be able fight. If not, the Romans would be outnumbered 200 to 30 when they boarded the pirate ship as the pirates rowers would be able to fight.
Factual error: Iceland sure looks a lot like the California desert.
Factual error: When the military are going to attack the flying saucers in Washington D.C., there's some shots where you can see trucks and airplanes in a deserted region. Washington is hundreds of kilometers away from the desert. (00:18:30 - 00:21:10)
Factual error: Karnak was an Egyptian city, not one of their gods.