Continuity mistake: At the beginning when Maximus is walking past the rows of soldiers look closely and you can see some of the same soldiers twice. (00:03:15)
Continuity mistake: Watch as the Germanian leader gets killed. A soldier charges and stabs him on his left side. When it cuts the soldier charges and stabs him again. (00:03:35)
Continuity mistake: At the very start of the film when we see Maximus for the first time, he is talking to his friend Quintus. Two boys run past, yet when it cuts to another shot they have vanished. We should have been able to see them at least running off in the background somewhere. (00:03:40)
Continuity mistake: In the beginning when Maximus says "Lean and hungry. Still nothing?" in which he gets the answer "Not a sign" from Quintus. Watch the soldier behind them handling the catapult. When it cuts that soldier is suddenly gone. (00:03:45)
Continuity mistake: When Quintus tells the soldier to move the catapult he is standing right behind Maximus. When it cuts, he is further away from Maximus. (00:03:50)
Continuity mistake: Before they attack the Germanians, Maximus says "Strength and honor" to Quintus and he replies. Then Quintus says the same to another soldier who is facing him. When it cuts the soldier is suddenly walking away. (00:05:10)
Continuity mistake: When Maximus says "On my signal, unleash hell" he takes off with his horse. Then he rides through a stand with soldiers and the soldiers gets up on their feet. Then it cuts to a close-up of Maximus riding and as the camera zooms out the soldiers gets on their feet again. (00:05:30)
Continuity mistake: Right before the attack on the Germanians when Maximus gets to his soldiers and shouts "Fratres.", you can see two soldiers walking right behind him. When it cuts to the soldiers shouting "Maximus.", the two soldiers behind him has suddenly disappeared. (00:06:15)
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: When the Roman soldier shoots the signal arrow before the attack against the Germanians, you can see his bow twist at least 45 degrees between the shots. (00:07:25)
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)
Continuity mistake: When Maximus chops the head off the German at the beginning of the movie, his sword is supposed to be stuck to the tree. However after he has chopped the head off you can for a second see the sword in his hand before we get to the shot where the sword is stuck to the tree. (00:09:40)
Continuity mistake: When the Germanian leader is stabbed the first time he holds his weapon with both hands. When it cuts it's only in his right hand. (00:10:25)
Continuity mistake: The weather during the battle against the Germanians changes almost every second: Hard snowfall, soft snowfall, almost sunshine, soft snowfall, etc. (00:10:35)
Continuity mistake: In the wagon, when Commodus gets up and sits down beside Lucilla, the sunlight coming over his shoulder changes between the shots. (00:13:00)
Continuity mistake: In the beginning when Commodus and Lucilla are talking and Lucilla says "For now, the first thing I shall do is have a hot bath." you can see she has her hand on her chest. When it cuts Commodus is suddenly holding her hand. (00:13:25)
Continuity mistake: When Commodus arrives and asks Aurelius if he has missed the fight, Aurelius' hair is blowing in the wind. When it cuts from behind Aurelius his hair is not moving. (00:15:50)
Continuity mistake: When Commodus tells Aurelius he will sacrifice a hundred bulls to honor his triumph, the snowflakes on his head differ between the shots. (00:15:50)
Continuity mistake: When Aurelius gets on his horse and Maximus is helping him, you can see Aurelius' arms are under his cloak. When it cuts to Aurelius saying "So much for the glory of Rome" you can see his left arm is uncovered. (00:16:30)
Continuity mistake: After the first battle when Maximus is washing his hands during the feast, you can see in the close-up of his hands that the wool sleeve on his left hand is thicker than in the other shots. (00:17:05)
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