Revealing mistake: When the emperor lies dead he is clearly still breathing. (00:35:15)
Continuity mistake: When Aurelius says "Enough with politics. Let's pretend I'm a father." to Lucilla, she smiles and looks down. When it cuts she is still looking down, but in the next shot she is suddenly facing Aurelius again. (00:20:20)
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: 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: At the end of the movie when Maximus is lying dead, and Lucilla asks who will help carry his body. Her bracelet on her arm is positioned differently than right before she asks the question when she is looking around at the crowd. (02:20:10)
Continuity mistake: In the final battle, there are about 20 or something Praetorians on the platform when it moves up. But when you see the platform coming up to the arena, there are suddenly enough to form the "tortoise" position; and then they have multiplied in order to arrange the circle around Commodus and Maximus (about 45 Praetorians). (02:13:05)
Audio problem: When Commodus and Gracchus are having their little debate in front of the senate and Gracchus says "Then perhaps Caesar will be so good as to teach us, out of his own extensive experience.". Then it cuts to a wide shot of the senate, and you can hear them laugh. However, none of them are moving their mouths, nor seems amused in any way. (00:59:15)
Factual error: When Maximus walks through the wheat field it is obvious (to a botanist) that the wheat shown is an octaploid variety which was developed in the 1950's for the Green Revolution. The type of wheat grown during Roman times can still be found in many areas too, so it's not like the filmmakers can't find it. (02:19:45)
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)
Revealing mistake: Watch closely during the first gladiator battle as the man who is caught in a net gets charged by a man with a spear and a man with a sword. First the man with the spear stabs him, but when the man with the sword stabs him you can actually see him holding the tip of the sword pointing to the side avoiding to hit him. (00:54:55)
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: 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: 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: The first time Maximus is about to fight as a gladiator we see him grabbing some dirt in his right hand and pouring it into his left hand. When it cuts we can see both his hands covered in dirt before he start rubbing his hands, as if he already has rubbed his hands in a previous scene. (00:52:55)
Continuity mistake: When Maximus has killed the Praetorian and got wounded himself, we see him getting down on his knees and leading his right arm to the wound on his left arm. At this time we can see the sleeve on his right arm is by his elbow. When it cuts to a close-up of his hand we can see the sleeve is down by his wrist. (00:39:30)
Continuity mistake: When Lucius descends the stairs to greet his mother and Commodus, there is a brief close-up sideways which shows, behind them, a banner, and two stones with hieroglyphs. There are in fact several Egyptian obelisks in Rome, that were taken there in antiquity. The only trouble is, it doesn't match with the long shot at all. (Commentary track confirms this was not shot on one existing location - only the four top steps of the stairs were really there). (00:57:31)
Visible crew/equipment: When Commodus enters Rome in triumph, he descends from his chariot to mount the stairs. A group of children with flowers rise. At the extreme right, modern scaffolding is visible for a half a second. (00:57:28)
Revealing mistake: In the shot of the discussion (literally) over the dead body of Marcus Aurelius (horizontal in the foreground), the "corpse" is still breathing. His belly is rising slowly up and down.
Audio problem: When they get in the gates at Proximo's after he has bought the slaves you hear him say, "I am Proximo" but his lips aren't moving when he says it. (00:48:20)
Continuity mistake: When Maximus, Juba and the other slave are eating at a table, the big slave pretends he's choking. Juba has his hand on the spoon, yet when it cuts to the next shot, his hand is no longer there. (01:36:30)
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