Aetius tells the Emperor he will take the 3rd Legion north into Gaul. During the march, during the battle, and later during the victory parade back in Rome, all the Legion's standards say "20th Legion" in Latin.
When Attila has killed his brother in a duel, he claims N'Kara and goes raiding villages. If you look closely, you can see that he's already carrying the 'sword of the war god,' although at that point of the story he hasn't found it yet. He finds it later, after N'Kara has died giving birth.
From the time Attila became king of the Huns to his death, he does not age.
Attila did not kill his brother Bleda in a duel a day after his supposed coronation as King of the Huns, as depicted in the film. Nor did Attila become king after his brother's death. Historically, after the death of their uncle, King Roas, in 434, both Attila and Bleda shared the Hunnish throne until Attila killed his brother in 445.
The Roman emperor Valentinian III did not kill Flavius Aetius after his victory over Attila. He killed the general one year after Attila's death, in 454.
Theodosius II, the Byzantine (Eastern Roman Empire) emperor, was not alive when Attila was shown to be poisoned. He died in 450.
General Felix shouldn't have been in the movie at all because he was long dead in the time in which the movie is set. He was murdered in 430 A.D. on the orders of Aetius, many years before Attila became a threat.
In the party scene where Aetius is showing off Rome to Attila, one can see a woman with her back to us reveal herself to a man and woman. Despite her veil, one can see her thong underwear. This type of underwear is a 20th century innovation.
Attila rides an iron-shod horse; iron horseshoes weren't in use as yet at that time.
In the orgy scene, Honoria is wearing a style of corset for her top which was not invented until several hundred years later.
In the party/orgy scene where Aetius is showing off Rome to Attila, we see a woman with her back to us reveal herself to a man and woman. Despite her veil, we can clearly see her thong underwear.
When Aetius first displays the Roman legions to Attila, the soldiers are clearly marching out of step with the beat of the drums. Also, at the wedding of Attila and Ildico, the beat of the music is very much out of sync with the dancing and hand-clapping.