Factual error: When a group of soldiers goes to Maximus' villa to burn it and kill his family, his son points them, saying in perfect Italian "Mama! I soldati!" I just don't understand why he speaks Italian in a movie in English, where people are supposed to speak Latin, in a province where this language was never ever spoken.
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.
Continuity mistake: When Hagen tests Maximus in the sword fighting, watch closely the scene behind them. At the time when Hagen strikes Maximus for the first time you can see gladiator behind taking a glass. At the time of the second strike you can see only a glass. When Hagen stopped his third strike you can see the same gladiator holding a glass and looking around. This time he is turned toward the camera and you can see a red spot on his chest.
Visible crew/equipment: When Maximus enters the arena before facing Tigris, the camera pans around the crowd. At one point, you can see a young Hispanic girl wearing a "scrunchie" holding her hair in a perfect ponytail, several bracelets and a watch, a necklace, a cut-off white t-shirt, and blue jeans. Most noticeable in full screen version.
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.
Factual error: When the execution squad is ready, Quintus says "fire". Of course this is incorrect, since you don't fire a bow and arrow, but "loose" it. The term "fire" came only with the invention of firearms. It's a common mistake in bow / arrow type situations. (Extended version only).
Factual error: The opening battle is wildly inaccurate. The Roman legions were trained to fight as a regimented force, and to maintain formation for mutual support. In the film, the formation collapses instantly upon contact with the enemy. Further, the Roman legions used spears called pila. Doctrine called for them to be thrown while the enemy closed. The Romans would then draw their swords and fight, while remaining in formation. Though the Romans are shown holding their pila in the opening scenes, they are never used against the barbarians, and we see no pila-riddled shields and/or corpses in the background.
Factual error: At several points in the movie you will see speeches to a large crowd. These take place at the Piazza San Pietro. This square however is late Renaissance. Connected with this is the columns you see in these scenes. They were designed by the Italian artist Bernini, and the film crew didn't even remove the statues of all the popes.