Revealing mistake: During the first gladiator battle there is a shot of Maximus stabbing another gladiator. When it cuts from behind that gladiator it is obvious that the sword is resting in his armpit before Maximus pulls it out. This happens a few shots after Hagen chops off his fellow gladiator's hand. (00:54:45)
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: 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: When Proximo is having his speech for the slaves as he says "And as your mothers were there at your beginning" there is a shot of a man throwing a spear at another man. Look at the spear as it flies through the air. It wiggles as if it is attached to a wire. (00:48:40)
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: When Maximus is being taken to be executed he escapes - when he whacks the last man round the face look closely and you see he slashes about a foot away from the soldier's face. (00:38:45)
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: When Maximus is riding against the Germanians, as he meets the first Germanian, he hits the Germanian's spear and hits him with his sword. However, the hit isn't even close, but about 30 centimeters from the Germanian. This is even seen without slow motion. (00:09:30)
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)
Revealing mistake: When Maximus goes up against the 6 gladiators by himself, the fourth gladiator (carrying an axe and wearing a large helmet) he slashes across the right side right above the belt. You can tell the slash doesn't cut, but leaves a trail of blood and in the next shot the blood disappears entirely. (01:06:55 - 01:07:00)
Revealing mistake: When the emperor lies dead he is clearly still breathing. (00:35:15)
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)
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: 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: In the beginning during the war with the Germanians when Maximus falls off his horse you can see that it's not Russell Crowe but a stunt double. (00:09:55)
Revealing mistake: When Maximus and friends are viewing the Rome Colosseum for the first time, the video raises to show the full length of the Colosseum. At the top birds are flying over it but some fly and disappear into nothingness. (01:18:44)
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