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.
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: 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.
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 Vision - S4-E12
Continuity mistake: Shots from inside the boat show all 3 boats without sails, but aerial views and shots from behind the boats shows them with sails. (00:36:30 - 00:38:40)
