The appearance of the Kattegat fjord changes through the season, with the fjord opening up and mountains disappearing.
In season 2, episode 2, Ragnar bids farewell to his wife before leaving, and then turns to her pregnant belly and says "And to you, Sigurd". Yet, later in episode 4 when he returns and Aslaug presents to him their son, he asks her "How did you name him?"
When Bjorn is a child, his eyes are brown. As an adult, they are blue.
Halfdan the Black and Harald Finehair were not actually brothers, but father and son.
In history, Norsemen dressed more extravagantly and possibly more provocatively than portrayed in the show. They dressed in bright colors, bathed weekly and used primitive hair-dyes.
The first Earl of Kategat is referred to as "Haraldson". In fact, the proper way of addressing a Norseman is by their given name, not the second name which is a patronym. This custom still exists in Iceland today.
Ragnar Lothbrok's sons are referred to as "Bjorn Lothbrok", "Ivar Lothbrok", etc. However, Lothbrok is a nickname (meaning "shaggy breeches"), not a family name. The sons should properly be referred to as "Bjorn Ragnarsson" (i.e. son of Ragnar), etc. Family names were not used in Scandinavia at that time, did not become common until 17th century, and were not standard until the 19th.
The role of the Seer would have much more likely been held by a woman (a völva) in actual Norse society.
Earl is used mostly as the title of Norse noblemen rather than the actual title jarl. This is technically not incorrect as earl is a loanword from the old Norse term jarl.
Most historian hold it very likely that the Norse had contact with the Anglo-Saxons in the later part of the 8th century and the existence of England was widely known to them, unlike how they are portrayed in this show. Contact and trade with Anglo-Saxons is actually a likely explanation for the Vikings ability to locate monasteries such as Lindisfarne.
Character names are fairly inconsistent through the series. Modern Scandinavian forms, Latin forms and old Norse forms are used invariably. For example, Kjetill is the modern Norwegian form of Ketill, with the "kj" pronounced with as the "ch" in German "ich". In old Norse it would be with a hard "k". In contrast, the character Egil the bastard has his name pronounced with a hard "g" like in old Norse (and some continental Scandinavian dialects) as opposed to the modern Icelandic which pronounces the name with a soft "g".
Bjorn's girlfriend is always referred to as "Porunn", yet the credits show her name, correctly as "Þorunn". The "Þ" character, an Icelandic letter known as thorn, is equivalent to "Th" so her name is really "Thorunn"
King Aella could not have reigned in Northumbria at the time of the Lindisfarne raid, in 793. His reign begun around 863 and ended in 865.
The Sami are seen wearing four winds hats. These hat appeared in the 18th century, a thousand years after the setting of the series.
Charlemagne is talked about as having been dead for quite some time. Charlemagne died in 814 and lived to witness the beginning of the age of the Vikings.
The whole time line is quite mixed up for the series. At the time of Lindisfarne (S01), Ragnar wasn't even born. King Ecbert died about 6 years before Ragnar, in 839. Aethelwulf would have been king at that time. Aethelwulf himself died in 858, 7 years before the invasion of the Great Heathen Army. In the series, the invasion seems to follow Ragnar's death almost immediately, while in reality there was a 20 year gap between both (his sons - with the exception of Bjorn - being young children when he died). Season 4 also shows the winter at Repton as if it were immediately after landing in England, while in reality it was 8 or 9 years after the invasion. Ecbert (who is shown as king of Wessex) and his son Aethelwulf were long dead. Also the successive kings Aethelbald, Aethelberht and Aethelred (sons of Aethelwulf) were dead. King at the time of Repton would have been Alfred, Athelwulf's youngest son, about 24 years old. Not a child as shown in the series.
The first Viking attack on Paris did not occur until 844.
The series shows the Uppsala Temple as being located in the mountains. Uppsala was and is located in the Mälaren Valley which does not have any mountains.
Kattegat seemingly is located in Norway, yet people are show riding between Kattegat and Hedeby, despite the later being located in southern Denmark and there being no land-connection between greater Scandinavia and the danish mainland until a 1000 years later.
Vestfold is shown as a mountainous coastal region while in reality it's highest peaks are far into the region and the coast of Vestfold is one of the flattest parts of Norway.
Hedeby is shown as being located in the hills, as opposed to a sea side trading town surrounded by swamps and farmlands. Also, Hedeby was located in the south of modern Denmark, which highest peak is about 172 meters.
Gotaland/Götaland seems to be located high up in the mountains. Through a large part of Götaland is covered by hilly highlands, the highest peak in Götaland is 362 meters.
In the series, the existence of England is unknown in Norway, yet Rollo can immediately identify Athelstan as a Saxon, despite never heard old English before or seen an a Northumbrian. The only Saxons he could have meet in that case would be the "old" Saxons of Saxony who were culturally and linguistically distinct.
Judith was the second wife of Aethelwulf, and wasn't the mother of any of his children, including Alfred. She was the daughter of the King of France, not Northumbria. And he was king of Wessex when he married her; she never knew her father-in-law Ecbert/Egbert.