Revealing mistake: Early in the film, when Hughes and another guy are walking past one of the replica WW1 aircraft used for the "Hell's Angel" scenes, you can see the ends of the cylinders of the modern air cooled engines they used to power it. The original aircraft (SE-5A) used a water cooled V8 which wouldn't have been visible.
Revealing mistake: Towards the beginning of the movie, young Alexander tames a horse and then jumps on the horse and rides off. He is supposed to be riding bareback, but in one of the shots from a distance, you can tell that the actor's feet are in stirrups.
Revealing mistake: In an early scene Cole and his future wife watch fireworks over Paris through a doorway overlooking a balcony. The explosions are reflected on the glass of the open door, which is at an angle to the POV. But the CGI fireworks and their reflection in the glass are squashed to fit into the foreshortened pane of glass. In reality the reflected image would be like any mirror, with the image inverted left-for-right.
Revealing mistake: When Britt and Peter are having sex, it is obvious she is wearing flesh coloured underwear.
Revealing mistake: While the soldiers are putting the corpses of Eva Braun and Adolf Hitler in the ground outside the bunker to be cremated, one of their legs moves.
Revealing mistake: In the opening scene, where Dwayne is standing on the boat, it shows he turned to his left. The next close-up, he is turning to his right.