Revealing mistake: In the "Battle of Carthage," towards the end just after the repeater crossbow has been fired, we see one of Maximus's fellow gladiators running towards one of the fallen chariot riders. You can clearly see the spike that he kills the chariot rider with moving as he runs, suggesting that it was made of a softer, safer material. (01:23:25)
Revealing mistake: When Juba kills the gladiator lying on the ground during the Battle of Carthage with his helmet, we can actually see Juba hitting the ground beside the gladiator's head. Then we see blood splash up from the ground beside his head. (01:23:25)
Deliberate mistake: In the fight scene with the chariots Maximus takes one of the horses from a chariot and jumps on it. The horse already has a saddle complete with stirrups despite the fact it was pulling a chariot and not carrying a rider. (01:23:30)
Continuity mistake: When Maximus is on a horse pursuing the chariot with the female gladiator throwing a spear at him and misses, we can see there is an extra spear standing on the chariot. Then Maximus throws his spear and when it cuts to the spear hitting the female gladiator the spear in the chariot is suddenly gone. (01:23:45)
Continuity mistake: After Juba has thrown the sword to Maximus, Maximus tries to gather the gladiators by shouting "Single column, single column," then it cuts to him charging on his horse while holding his sword arm out. When it cuts from behind him he is holding his sword arm in front of him instead. (01:24:10)
Continuity mistake: During the fight with the gladiators in the chariots, Maximus jumps on a horse and chops off a gladiator's head. Now you can clearly see the blood on the blade. But when the fight is over before he salutes the crowd, you can see the blade is clean. (01:24:20)
Continuity mistake: During the battle of Carthage there is a scene where Juba jumps on to the back of a moving chariot. When it cuts the chariot is suddenly standing still, Juba is standing on the ground while his mates are grabbing the gladiator on the chariot. (01:24:30)
Revealing mistake: As the last man is killed during the Battle of Carthage, you can see that he actually doesn't get stabbed. The sword hits the ground beside him. Furthermore, he doesn't bleed at all. (01:24:35)
Continuity mistake: At the end of the first battle in the Colosseum, Maximus puts his sword up to the crowd. There is blood at the tip of the sword, but in the following shot, it has vanished. (01:24:40)
Continuity mistake: During the enactment of the Battle of Zama, Maximus rides his horse around a bend with his sword in his left hand. When we cut to close up, it's back in his right. Looking at the left-handed shot in freeze-frame, you can see the frame has been flipped, left-to-right (check the armor). This was probably for continuity reasons. Similarly, Maximus jumps on the horse and his shoulder protector is clearly on his injured left shoulder. However, a few frames later, the protector is on his right shoulder, then it moves yet again back the correct spot, the left shoulder. (01:24:45)
Continuity mistake: When the gladiators have won inside the coliseum for their first battle, Commodus goes to see Maximus in the arena. In the previous shot, Maximus gets off of a white horse, yet in the following shot, the white horse is at the bottom of the screen with Maximus and the other gladiators some distance away. (01:25:35)
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: After the first battle in the Colosseum (The Battle of Carthage), Commodus decides to meet "The Spaniard" in the arena. When Quintus (the head of the Praetorian Guard) is telling the slaves to drop their weapons, we can see one of the spectators hiding a plastic bottle of water (right upper corner of the screen). (01:25:58)
Continuity mistake: At the end of the first battle inside the Colosseum, Commodus goes out to see Maximus, yet when he first enters the arena's floor, the distance from him to Maximus changes three times during shots. For example, when he first goes through the gate, it cuts to Maximus, then back to Commodus and he is just coming out the gate again, yet in the previous shot he came out the gate. (01:26:15)
Continuity mistake: During the first gladiator fight in the Colosseum, Maximus picks up the arrow from the ground. His hand is covered in blood, but in the second camera shot, no blood at all, and then in the third shot, you can see the blood again. (01:26:25)
Continuity mistake: After the Gladiators are victorious at the Battle of Carthage, Maximus reveals his identity to Commodus. The time seems to change from sunset to high noon a number of times. The crowd shots are at noon, the close ups are clearly made with the sun on one side of their faces. (01:27:20)
Factual error: In the arena, Maximus tells Commodus and the Roman spectators that he is "general of the Felix legions." The problem is, there was just one Felix legion, namely Legio IV Flavia Felix. (01:27:35)
Revealing mistake: When the chariot crashes in to the gladiators, we can see the shield in which the chariot crashes in to. It's not shaped like the other shields which are shaped in a perfect bow. This shield is rectangular shaped most likely to balance better on what ever is supporting it when the chariot runs over it making the chariot tip over. (01:30:00)
Audio problem: When Commodus is talking with Lucilla before the Battle of Carthage as he is saying "I could announce the dissolution of the senate...". It cuts from behind him and he says "...are the people ready?", however his lips don't synch up with what he says. (01:32:25)
Factual error: During a meal in the gladiatorial school, Juba is shown eating with a metal spoon. Slave-gladiators almost certainly only ate with their fingers, wooden spoons, or wads of bread to sop up their food. They never would have access to metal spoons that could be sharpened and used to attack the guards keeping watch over them. (01:36:20)
Chosen answer: No, they are full brother and sister. They both had the same mother and father. Incest was not exactly normal at that time, but it stretches back within the imperial families as far as Caligula and possibly earlier. There is no evidence to suggest that the real Commodus was attracted to his sister, it was probably just included in the film in order to make the character seem more disturbed, and also as another reason why he would be Maximus' enemy (Maximus and Lucilla were, after all, once lovers).
The Doctor