Factual error: Early in the film Maximus walks through a cereal crop trailing his hands against the heads of grain. Except that dwarf varieties of cereal were only bred in the 20th century. Before this a crop would have been up to a man's shoulders.
Suggested correction: No doubt the scene was shot with a modern variety, but it turns out that it is a good approximation of the wheat grown at the time of the story. Two varieties of wheat grown in ancient Rome, the Emmer and Eikorn varieties, reach only 2-3 feet at maturity. These were originally wild wheats that had been cultivated for 8-10 thousand years BCE.
Factual error: Several men, most notably Marcus Aurelius, are long-haired (for male standards) in the movie. In Roman fashion, men were expected to use their hair short. Male long hair was seen in a very bad light, especially for an emperor.
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)
Visible crew/equipment: When Commodus enters Rome in triumph, he descends from his chariot to mount the stairs. A group of children with flowers rise. At the extreme right, modern scaffolding is visible for a half a second. (00:57:28)
Revealing mistake: In the shot of the discussion (literally) over the dead body of Marcus Aurelius (horizontal in the foreground), the "corpse" is still breathing. His belly is rising slowly up and down.
Continuity mistake: When Proximo gives Maximus his armor the horses have wings. After that they do not.
Revealing mistake: When Maximus and friends are viewing the Rome Colosseum for the first time, the video raises to show the full length of the Colosseum. At the top birds are flying over it but some fly and disappear into nothingness. (01:18:44)
Other mistake: When Proximo says "Use this" to Maximus and throws him a piece of body armor, it is not the same armor he uses in the scenes that follow. (01:10:00 - 01:12:55)
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.
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?"
