Audio/visual unsynchronized: As Romulus passes Caesar's guard in the coronation procession (at 00:09:56), Demetrius mouths the words "We're dead." (In the DVD commentary it is explained that "we took it out just because it was too soft to be heard over the noise of the procession and the music.")
Factual errors: The sword the legendary king Arthur pulled from the stone was not Excalibur, he pulled a sword from the stone in the legend but that one later broke in battle, so he had a new blade forged: Excalibur. It would not make sense to most viewers however, if it wasn't Excalibur in the stone.
Revealing mistakes: When Wulfila's finger is about to be sliced off, the knife put up against it is placed blunt/dull side towards the finger, thus making it impossible to slice off the finger.