Continuity mistake: During the enactment of the Battle of Zama, Maximus rides his horse around a bend with his sword in his left hand. When we cut to close up, it's back in his right. Looking at the left-handed shot in freeze-frame, you can see the frame has been flipped, left-to-right (check the armor). This was probably for continuity reasons. Similarly, Maximus jumps on the horse and his shoulder protector is clearly on his injured left shoulder. However, a few frames later, the protector is on his right shoulder, then it moves yet again back the correct spot, the left shoulder.
Visible crew/equipment: During the scene where Maximus throws the sword into the crowd and yells "are you not entertained," the camera pans around, and in the crowd, for a split second, you can see a cameraman in jeans and a shirt standing next to a camera. This is visible on the big screen, but hard to see on video, unless you see it on DVD and can zoom in.
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.
Continuity mistake: When we get to the comedy play fight between Maximus and Commodus, we look at the front of the stage and we see the actor playing Maximus is standing in the middle of the stage and the one playing Commodus on the right side. When we get a view from back of the stage 'Maximus' is standing on the left side of the stage and 'Commodus' in the middle. They have suddenly switched sides.