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).
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)AmberMac420
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)