When Athelstan tells Ragnar the priest next to him has died, the priests hands are tied at the wrist. But when they thrown him overboard his hands are not tied when he hits the water.
After the dead priest is thrown overboard Athelstan says a prayer and crosses himself with his left hand. A priest/monk would know you always cross yourself with the right hand.
In the first shot after coming back from Lindisfarne, the camera passes over the treasures in the Earl's house and there is a modern screw on the nearest artifact.
While the episode is set in in 793, King Aelle is said to be the king of Northumbria. King Aelle ruled in the later part of the 9th century and died on 21 March 867.
At 5:36, as Ragnar's ship is traveling inland; a drone can be seen coming into shot from behind the hill on the left.
At 26 minutes in, when Ragnar returns home, a modern red cart can be seen behind his house.
Athelstan tells Ragnar that before his raid, the abbey at Lindesfarne had no need to be defended from raiders. Yet when the Vikings first show up the monks immediately realize they are raiders and know to take refuge.