Athelstan is often seen wearing a rosary around his waist while in the service of King Ecbert. This devotion's earliest form only started in the mid 12th century and would have not been known to Athelstan.
Prior to the Second Vatican Council the Priest celebrated the Mass with his back to the congregation. During the Consecration the Bishop is shown facing the congregation.
During the rainstorm, Ragnar is seen eating at the table. When he lifts his cup, there appears to be a UPC sticker on the bottom.
During the battle with jarl Borg, a man in a blue t-shirt and shorts is seen running up a hill in the background.