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)
Continuity mistake: During Maximus's death scene at the end you can clearly see the entrance that Commodus and Maximus entered the arena through in the floor of the arena. After Juba is done burying the figures of Maximus's dead son and wife, you can not see the entrance anymore when he is walking away. (02:21:00 - 02:22:55)
Continuity mistake: When Proximo is about to be killed and says "shadows and dust", the background scenery changes. It is obviously an alternate take from when he says the same thing before Maximus fights Tigris, used because of the death of Oliver Reed, but that doesn't make it any less a mistake. (02:12:15)
Continuity mistake: After Maximus comes out of the tunnel after leaving the gladiator quarters, he sees his friend Cicero. Maximus makes a bird noise to get his attention, Cicero sees him and in an attempt to warn him he yells "Maximus." He startles his horse which runs away but a noose is around his neck so he is pulled off the horse and left hanging there. As soon as the horse leaves him and he starts swinging his hands you can see they are not tied. Then when Maximus runs up to him and holds his feet up to try and save him his hands are tied up. (02:12:50)
Continuity mistake: In the Gladiatorial fight scene in Morocco, Maximus throws one sword with his right hand into the stand, still holding a sword in his left hand. The next instant long shot of him shows the remaining sword in his right hand as he taunts the crowd with "Are you not entertained?"