Deliberate mistake: During the Trojan attack on the Greeks at the beach, before Hector and "Achilles" have at it, there are two separate close-ups of Hector's sword slicing across a man's face, who promptly spins around and spews blood. This particular man is enjoyed twice in two takes of the same shot, just before and after Odysseus' close-ups. Then as if that's not enough, a few shots later this spewing man is actually seen a third time, but this time from another angle. (01:42:30)
Deliberate mistake: When Arthur follows Guinevere to the meeting with Merlin, her and the trees' shadows point at the viewer. Only Arthur has two shadows, one points to the left. The source of the moonlight is below the next ridge, and it's bright like a floodlight. Later, when Arthur is talking to Merlin, his face is always illuminated, even after a 180°-turn. (00:59:00)
Deliberate mistake: When Denzel Washington is in the bathroom, he discovers the camera monitoring him from the air vent, by seeing the light on the front of the camera. Any spy with common sense would not install a hidden camera with a light on it.
Deliberate mistake: When Alexander is giving his pre-battle speech at Gaugamela, some of the shots are 'flipped', which has the odd effect of suddenly making all of the phalanx pikemen left-handed.