Continuity: Towards the end, when Einar climbs the tower to get to Morgana. One of the metal 'flowers' on the back of his tunic is obviously missing. However in later shots, it is back in place again.
Factual errors: The end credits are based upon the Bayeux Tapestry, a record of the 1066 Norman Conquest of Britain. Although the Normans had Viking forbears, such a was not at all representative of native Viking art.
Anachronisms: A Norman-style stone castle is featured in England, though the film is set before the Norman Conquest of 1066.
Crew or equipment visible: In the early scene where the viking ship is first returning to the village a shot from the front of the viking boat (from the camera boat in front) clearly reveals the camera boat's wake trailing behind - but in front of the viking ship.
Continuity: When Einar jumps into his small rowboat at night, the boat is still tied to the wharf, yet in the next shot it is untied and sailing properly.
Anachronisms: The Northumbrian flag, a complex banner flying from the tops of Aella's castle towers, would not have existed in this period. The Viking age preceded formalized heraldry by centuries. Northumbria did have a flag, as many pre-heraldic kingdoms did, it was a counterchanging banner of 8 vertical stripes, red (or purple) and gold. It is also unlikely that Aella's soldiers would've had shields bearing heraldic devices. If the shields (a red 'X" on a white background) were meant to signify St. George's Cross (which today stands for England in the UK's Union Jack), that is also anachronistic. The symbol was not used until centuries later and it was a cross (vertical & horizontal members) instead of a saltire (diagonal members).
Continuity: When Erik rescues Morgana and her maid they get into a rowboat with the two men near the bow and the two women in front of them. There is even a whole sequence when Erik rips the back of Morgana's dress open. A few seconds later they have switched positions and the women are near the bow of the boat with the men in front of them.
Audio/visual unsynchronized: When Ragnar and Einar are celebrating the capture of Morgana, Ragnar is heard to say 'What a son'. This line is re-used from the earlier hatchet throwing scene.