Revealing mistake: In the "Battle of Cartage" when the first of the chariots is turned over, right after Maximus is saying "Hold, hold...down low, down low," there is a series of shots to the crowd, there is a bearded man celebrating with both arms, and deep to his right (left on the screen) there's another man, wearing some kind of clothes with stripes black and white. Behind him, there is an old man who appears very briefly, and he's wearing glasses. (01:25:50)
Revealing mistake: During the gladiator battle where Maximus is fighting gladiators alone, there is a particular shot with the gladiator he recently sliced up the stomach on. First Maximus hits him in the face, and you can see blood gutter up from the ground instead of from his face. Then when it cuts to Maximus stabbing him, blood gutters from behind Maximus, when in fact the gladiator is standing in front of him. (01:04:20)
Factual error: As the Gladiators are being led into the Colosseum for the first time, an elephant is led across the background. Although the Romans used elephants in the arena, they used African elephants, and the one we see is an Asian elephant.
Continuity mistake: Before Lucilla gets to Commodus who asks her if she knows if Gracchus has a new lover suspecting her to be his new lover, she walks by two servants. The servant on the left is looking down with his head bowed. When it cuts he is suddenly standing up straight looking after Lucilla as she walks by. (Extended version only). (02:08:35)
Continuity mistake: Just before the battle with against the Germanians begins, while the Germanians are taunting the Romans, there are many arrows in the trees and in the ground. Twice we see this. Problem is that the disciplined Romans had not fired a single arrow at that point and no skirmishers, which could have been authorized to shoot before the order to "loose" can be seen in the open area in front of the defensive position. (00:06:50)
Continuity mistake: At one point during the first Germanian attack you see the Roman soldiers creating a line-up with shields, soon after followed by the Germanians running towards them off-hill (starting their attack). Clearly a few Germanian stopped fighting a little too early and you see them (left-side) turning and looking around. (00:09:15)
Revealing mistake: In the Moroccan prison the iron bars are electric arc welded. As a blacksmith, I can easily spot the visual differences between electric arc welding (a modern technique) and pierced bar joinery such as the Romans would have used.
Character mistake: Prior to the opening battle, Quintus says "The danger to the calvary" instead of "cavalry." (00:04:00)
Continuity mistake: During the final scenes, when Maximus had died and his body is lying on the sand, we can see both the slaves and the Praetorians forming a semicircle around Maximus. When Quintus asks for help to carry Maximus body, during the second it takes long to raise it up (shot change), the slaves have moved and the Praetorians are forming a perfect honor corridor through which Maximus's body is about to go by. (02:20:55)
Continuity mistake: When Commodus goes to see his father thinking his father would pass his powers onto him, if you look closely when he enters, there's a white sheet blowing rather vigorously in front of the camera. When it cuts to a shot of Commodus, the sheet now is hardly blowing at all. (00:30:35)