It is one of my favorite movies, if not the most favorite, but as a history enthusiast I am curious what were the biggest historical errors.
Love this movie, here are a few off the top of my head
Commodus did not die in the arena, although he did love gladiatorial combat. He was eventually assassinated in 192 AD (strangled in the baths).
Commodus also gets a pretty bad presentation throughout the film, including him being shown to have murdered his dad. This didn't happen.
The whole idea of restoring Rome to it's Republic beginnings is not true. Commodus had been groomed to take over upon the death of his dad. The senate was not going to be given the same powers it once had upon his death.
These are a few things from the top of my head!