Continuity mistake: In the opening scene 2 druids are talking to the archdruid; one is standing behind the other. In the next shot they are standing side by side. (00:02:00)
Continuity mistake: When the young Vercingétorix arrives at Gergovia and tells his uncle that he is returning the horse, the blankets on the horse's back change position between shots. (00:04:00)
Revealing mistake: When Vercingétorix leaves the Romans to go to Gergovia there is a stone mason by the road carving what is probably a distance marker. The man's chisel is obviously not in contact with the stone - while he is banging it with the hammer it is waving around in the air. (00:17:00)
Continuity mistake: When we see Caesar, in the town of Bibracte, asking why the archdruid has not been found, in some shots his robe is on his shoulder and in other shots it is not. (00:19:20)
Audio problem: When Vercingétorix tells Caesar that Alexander the Great was dead by the time he was Caesar's age, his mouth movements don't match the words. In this shot he is, I think, speaking French with the English dubbed over - he looks like he is saying 'mort'. (00:24:50)
Continuity mistake: When Vercingétorix is at Avaricum, saying he needs to burn it to prevent the Romans getting the supplies, at one point the people behind him all change position. (00:43:55)
Other mistake: One of the shots of the aftermath of the battle is played in reverse - e.g. the smoke from the fires is going down. (01:41:20)