- Roman cavalry using stirrups, which wouldn't appear in the West for another few centuries.
- speaking of horses, the horses Romans used were very small by today's standards; not the big bays used in the film.
- The Emperor and his entourage wearing deep bluish-purple colors to denote royalty. 'Purple' in Ancient Rome, was closer to what we today call maroon.
- Young Lucius' hairstyle was appropriate for a beach party in the 1970's. But in Ancient Rome, his hair would have been clipped short.
- Nobody would have used the term "Senator Gaius". 'Gaius' was a given name, his praenomen. He would have been referred to by his nomen and cognomen out of respect, or just his cognomen among friends.
- Maximus would not have been submissive or deferential to the senators in his camp. As a Roman general, even as a Spaniard, he would have been of the Senatorial class, and therefor the same rank. In fact, his auctoritas would have been higher than theirs, as a successful field commander talking to civilians.
- The Praetorian Guard did not wear the highly fascistic black armor depicted in the film. They were proud of their shiny, expensive, highly-polished armor that was a privilege of the job.
- Commodus ruled for eleven years, not the year or so depicted in the film, and was highly popular in his day, at least with the common people.
- Nearly every detail of the emperor's palace in the film is wrong. I'll leave it at that.
OTOH, they got an awful lot right, but I'll save that for another time... 
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