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)
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)
Revealing mistake: When Hagen pretends to choke on the beans, just after he finishes laughing, there is a shot of him sitting between Maximus and Juba. You can see his non-Roman underpants. (01:36:35)
Revealing mistake: As Tigris lays injured on the floor of the Colosseum, there are several close-ups of Maximus holding the axe above his head, with light streaming in around him. There are two things wrong with this; First of all, the camera angle is nowhere near steep enough to see the top of the Colesseum, therefore no light rays. Secondly, the top of the set can actually be seen between the light rays, as the canvas shades are too short and wide to be those seen in the CG wide-shots. Use the DVD "Making Of" as reference. (01:43:20)
Revealing mistake: When the firesquad has executed the two prisoners it cuts to a closeup of Lucilla. Later when it's time for Maximus to fight Commodus the exact closeup shot of Lucilla is being used over again. (Extended version only). (01:47:15 - 02:29:10)
Revealing mistake: In the scene where the Praetorians attacks the gladiator's quarters and the gladiators are forced to fight in order to hide Maximus' escape, Hagen is shot full of arrows. Note the blocky silhouette of his torso area? What kind of padding is he wearing under that canvas jerkin? (02:07:10)
Revealing mistake: After Hagen has been shot and stabbed, as he falls dying to the ground, the pad the arrows are attached to on his back is quite noticeable under his shirt. (02:07:15)
Revealing mistake: Near the end of the movie, when Commodus and Maximus are in close, struggling for the knife, take a good look at the knife. Both of the actors' hands are gripping it and it's clearly rubber because it's bent at a serious angle. (02:16:30)
Revealing mistake: When the Roman soldiers throws bowls of oil on the servants and set them on fire, the protective mask on the burning servant standing up is visible. (Extended version only). (02:19:00)
Revealing mistake: After Maximus is dead, if you look at his neck, you can see his pulse. (02:19:15)
Revealing mistake: At the end, when Maximus had gone to afterlife world of Elysium and was meeting his wife and son, both the wife and the son put their hands up as to shield the sun from their eyes. You can clearly see that their hands are not blocking out the sun, because the shadows are not over the eyes. (02:19:45)
Revealing mistake: When Maximus is dying and entering Elysium, you can see that it's not Russell Crowe walking through the fields to his wife and son. The stand-in's build and physique is totally different. (02:19:45)
Revealing mistake: When Lucilla takes her hand and closes Maximus's eyes after he dies you can clearly see him blink. (02:19:55)
Revealing mistake: After the final fight scene, where Maximus is dead and on his back in the sand, his armour would have made his head droop horribly backwards. To avoid that, the filming crew put a pillow of sand under his head to raise it about three inches. (02:20:10)
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.
Revealing mistake: During the first battle the Romans had with the Germans, they show a scene in slow motion with very emotive music that shows the massacre in the forest. Look closely at all the Romans and you will see one fall to the ground, then, as if it was a rewind, come back up again - and then fall again.
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.
Revealing mistake: In the opening battle, it's quite obvious in close-up the arrows shot by the ballistae (war machines) can't really be travelling more than a few yards at most, not the great distance they're supposed to be flying and which real ballistae could manage.
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