When Freydis stabs Christian and says "Now it's my turn to give you a cross", he already has the horizontal slash of the cross on his chest.
Harald Sigurdsson (Harald Hardrada) could not have been in England at the time of the St. Brice Day Massacre which occurred in 1002, because he was born in 1014 or 1015.
When Vikings arrive in Kattegat, a guy in the market is shouting in Farsi and advertising Persian Isfahan carpet which is weird considering that they were not popular until at least 16th century.
When Vikings arrive in Kattegat, a guy in the market is clearly shouting in Farsi and advertising Persian Isfahan carpet which is odd considering that no one spoke modern Farsi at that time, and Persian carpets were not popular until at least 16th century.
Aethelred II the Unready, is portrayed here as an old man by an actor who would be around 71 at the time of filming. In reality Aethelred was only around 36 at the time of the St Brice's day massacre in 1002 and died when he was aged about 49 in 1016.