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.
Continuity mistake: Maximus is about to be executed in the forest in Germania. Maximus kneels on the floor. The swordsman steps behind him and lifts the sword above his head to drive it down into the back of the neck of Maximus. When Maximus stands up and head-butts the swordsman the swordsman's arms are now by his side and the sword gone. The next shot shows the arms back in the air, with the sword, and Maximus grabbing the sword. (00:38:35)
Revealing mistake: After the first battle in the Colosseum (The Battle of Carthage), Commodus decides to meet "The Spaniard" in the arena. When Quintus (the head of the Praetorian Guard) is telling the slaves to drop their weapons, we can see one of the spectators hiding a plastic bottle of water (right upper corner of the screen). (01:25:58)
Revealing mistake: As Tigris lays injured on the floor of the Colosseum, there are several close-ups of Maximus holding the axe above his head, with light streaming in around him. There are two things wrong with this; First of all, the camera angle is nowhere near steep enough to see the top of the Colesseum, therefore no light rays. Secondly, the top of the set can actually be seen between the light rays, as the canvas shades are too short and wide to be those seen in the CG wide-shots. Use the DVD "Making Of" as reference. (01:43:20)
Factual error: Architectural elements were from several centuries later. Note the balustrades, bell towers and domes in the Rome skyline shot and, more than anything else, Maximus' house, a typical 15th century Chianti villa. Also the columns are not painted, which they were in ancient Rome. (00:43:10)
Revealing mistake: In the scene where the Praetorians attacks the gladiator's quarters and the gladiators are forced to fight in order to hide Maximus' escape, Hagen is shot full of arrows. Note the blocky silhouette of his torso area? What kind of padding is he wearing under that canvas jerkin? (02:07:10)