Revealing mistake: In the beginning of the film at the major fight sequence. When you see the Romans shield themselves with the rectangular shields against the barbarian's arrows and then they charge at each other. If you look closely enough, you can see two Roman soldiers standing there having a great laugh, one pointing a sword smiling, and the other one staring at the ground laughing to his heart's delight. (00:09:15)
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: When Maximus charges his cavalry against the Germanians in the first battle, he decapitates a Germanian standing in front of a tree. In the side-on shot of this, the Germanian is obviously a mannequin, as when the head comes off, so does the wig on top of it. (00:09:40)
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: In the shot after the Germanian leader get stabbed to death there is a Roman soldier and his horse falling to the ground. Look at the soldiers around him. They are moving unnaturally fast, like they are in fast forward. (00:10:35)
Revealing mistake: When the emperor lies dead he is clearly still breathing. (00:35:15)
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: As Maximus approached his home after he should have been killed, the grain fields clearly show marks which are only caused by a tractor during crop spraying. (00:42:00)
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: 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: 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: After the first gladiator fight when Maximus and Juba have finished off the last gladiator, it cuts to a wide shot of the arena. The camera moves to the left, if you look at the back row you can see two men with modern day T-shirts talking. (00:55:45)
Revealing mistake: When Maximus quickly kills all his opponents, the third to last man he kills of the group of men, when he stabs him in the stomach, you can clearly see him "stab" to the side of his stomach. Even more visible when you slow it down. (01:04:20)
Revealing mistake: During the gladiator battle where Maximus is fighting gladiators alone, there is a particular shot with the gladiator he recently sliced up the stomach on. First Maximus hits him in the face, and you can see blood gutter up from the ground instead of from his face. Then when it cuts to Maximus stabbing him, blood gutters from behind Maximus, when in fact the gladiator is standing in front of him. (01:04:20)
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 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 "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: 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)
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)
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