Revealing mistake: When they are spearing fish near the start, a water drop goes from the water to the spear, showing that it is reversed. (00:04:34)
Continuity mistake: After spotting Lagertha's boats, Torvi points at the boats and lowers her pointing arm. Another shot later and she is still pointing and lowers her arm again.
Factual error: The snakes in the King of England's pit are all constrictors. They do not have venom and are too small to see a human as prey, yet they show a man dying from a bite by one of these snakes.
Factual error: Several of Ragnar's relatives are called "Lothbrok" through the series (most notably, Rollo and Ubba), but Lothbrok was not a family name. The vikings did not use family names at all, though they did use personal sobriquets (such as "Ironside", "the Boneless" or "Finehair"). "Lothbrok" was Ragnar's nickname only. A more appropriate sobriquet for any of his sons would be "Ragnarsson" (son of Ragnar).
The Vision - S4-E12
Revealing mistake: Two oarsmen (or rowers) in Ragnar's boat, the first in front of us and the second from the back, pretend to have an oar but have none. (00:38:35)
Factual error: A crosshead screw is attached to Ivar's frame. (00:11:21)
Factual error: During this episode, Ragnar is lying down with a half-eaten pear on his chest. That pear is of the variety Conference, it was developed by Thomas Francis Rivers in England in the late 1800s, so almost one thousand years later than the events of the show are supposed to take place (in the 800s). (00:38:00)
Factual error: Bjørn is living in a log cabin in the mountains. This logging technique was not common until 2-300 years later.
Revealing mistake: The battle has Lagertha fighting skeletons, but on her final skeleton she swings her sword downwards and massively misses it by a large margin, but it still falls.
Continuity mistake: They take away the ropes around his neck. When King Aelle start to hit him, the ropes appear around his neck again, when the shot changes to shortly after the hit, the ropes are gone again. (00:29:00)
The Profit and the Loss - S4-E7
Continuity mistake: While speaking with Wigstan, King Ecbert's left hand is open and slightly pointing at Wigstan. The next shot his hands are clasped in front of his chest.
Continuity mistake: Ragnar is using a shield for target practice, throwing three knives. The positions of the knives as they are thrown into the shield changes between shots.
Continuity mistake: In the shot where Ragnar asks Floki if he can build something to lift the boats over the mountain after Floki says "Everything I do Ragnar, is for you" it then shows Lagatha and than Ragnar points at Floki and Ragnar says "Let's empty the boats." The camera then shows King Harold and you can see Rollo standing behind King Harold when he should be with the French. (00:10:33)
Suggested correction: It is not Rollo.
Continuity mistake: Rollo and Bjorn are seen practicing with one another. Rollo slashes Bjorn across the left side of his stomach first and then slashes his right arm. After Rollo forces Bjorn into submission on his back he then extends his hand and helps Bjorn to his feet. When Bjorn stands up the slash mark across his stomach has completely vanished. (00:07:25 - 00:08:20)
Continuity mistake: When King Ecbert's Soldiers are coming out of the gate for the hostage exchange, the 2 soldiers on the right do not have shields. When the camera cuts and comes back to them, all of a sudden the shields appear. (00:09:43)
On the Eve - S4-E19
Continuity mistake: When Torvi shoots the man that was running toward her, the man falls and the arrow is in his chest but in the next shot the arrow disappears. (00:03:22)
The Message - S5-E7
Revealing mistake: When Floki and fellow Vikings march across Iceland, an aerial view of Dettifoss waterfall is shown, including the visible tourist paths, fencing and a small stone bridge on the left, used by tourists to visit around the site. (00:31:15)
The Message - S5-E7
Factual error: When Floki lands in Iceland with the other Vikings brought from Kattegat, they land on the beach near Dyrhólaey, the southernmost tip of Iceland. Moments later, they are shown walking near the waterfall of Dettifoss, on the opposing side of Iceland. That is almost 300km (186mi) away, assuming they walked on a straight line across mountains, glaciers, valleys and a highly inhospitable desert in interior Iceland. If they went along the coast, that's more like 625km (390mi). (00:30:55 - 00:31:16)
Factual error: In the scene, Floki walks towards a small waterfall today known as Kvernufoss. Floki is supposed to be the only inhabitant if not the entire Iceland, at least the region, as we are shown no sign of civilisation whatsoever, and as historical record tell us about Iceland in the early 800s (as the events of Vikings seem to take place). But a well beaten footpath is visible, leading to the waterfall at the end of the canyon. That path is an oft-travelled path tourists follow nowadays to reach Kvernufoss, a well-known site in the area. A path like that in Floki's time would simply not have existed (there is nothing in that canyon but that waterfall, no reason to go there if not for sightseeing) and represents a (tiny) intrusion of our modern world in a show set in the middle age Iceland. (00:37:42)
The Profit and the Loss - S4-E7
Factual error: When Floki has a vision of Aslaug outside the viking camp along the seine river in France, the vegetation is that of a pine forest and the landscape is that of a mountainous region. There is no such area from the English channel to Paris, which is only flatland. The scene was filmed in Scotland. (00:34:25)