Factual error: Women were not allowed to be mixed with men. Only Vestales could stay where men were. Women could see the games just from the last ring-level of the Colosseum.
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.
Continuity mistake: When Maximus is on the cart and the slave treats his wound, it's on his right arm, but in the line up with Oliver Reed, there is nothing on his right arm.
Suggested correction: The mark on his shoulder is there, but it's in shadow most of the time, so it's difficult to see. It can be seen just as his shoulder appears on screen at the 36-second mark of this YouTube clip. https://www.youtube.com/watch?v=ut7Df7ttMoE.
Suggested correction: This movie is not based upon facts. Many element were fictionalized. This is just example of fictionalized things.