At the start when Athelstan reads the Latin scrolls about Caesar's battle strategy, King Ecbert is seen moving a chess piece.
The Europeans had to wait until 15th century to known about the game, where as the events in this episode are from 10th century.
Aethelwulf's wife Judith is portrayed as the daughter of King Aelle of Northumbria. In real life, Judith was the daughter of Holy Roman Emperor Charles the Bald. The show also makes no mention of Aethelwulf's first wife, Osburh, the mother of all of his children, including Alfred the Great.
In the battle several of the Saxon warriors are shown wearing helmets of a 15th Century French/Italian design. Identical helmets were used by soldiers in the TV series 'The Borgias'. Looks possible that the costume department for 'Vikings' acquired the helmets from this source.
When Rollo is back at Kattegat he is treated for his injuries. This raises two problems. One, there is no reason why he was not treated before at King Egbert's villa with the other wounded soldiers and, second, considering the elapsed time, his wounds, left untreated, would most probably had gangrened.
King Eckbert's troops are wearing helmets from the Eighty Years War.
If one listens closely just after Bjorn greets Princess Aslaug after coming back from England (42m 44s), a camera shutter can be heard.