Continuity mistake: When Acacius arrives in Rome and meets with the Emperors, there is a man with a monkey on his shoulder, his face surrounded by bright light. An instant cut later, his face is in the shadows.
Continuity mistake: When Lucila reads two tablets, her right hand suddenly holds a stylus between instant cuts.
Revealing mistake: Before the big battle between Lucius and Acacius begins, the soldiers arrive at the bleachers, where we see several shots of the people. Check the crowd, and you'll notice it's a flipped shot of the previous people. (01:33:32)
Continuity mistake: Glyceo grabs the sword on the arena floor and extends his right arm. An instant cut later, his arm is lowered, then it cuts back to the previous angle of his arm extended.
Continuity mistake: When Lucius asks the doctor to give him the prison keys and send a message to the soldiers at Ostia, Lucius' arm is bent around the prison bars, except for a brief shot where it's suddenly straight; then the shot changes and it's bent again.
Continuity mistake: After Lucius quotes Virgil, he kills a man who collapses on a chair. When he lies dead, there's a leg or armrest on the floor parallel to his body. A shot later, it's perpendicular.
Factual error: Much like its predecessor, this movie sorta speedruns through emperors; Geta and Caracalla here are shown as both ruling Rome as "twin emperors" in 200 AD, and die within a couple of days of each other, betrayed by Macrinus, who dies the same day. None of that is anywhere close to how things happened, to the point that discussing dates is sorta pointless; nothing resembles history if not in the loosest sense. Macrinus in this movie is never even formally proclaimed emperor, as opposed to him ruling for at least a full year.
