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: 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: 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: 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 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 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.