During the approach of Lagertha's force to the city gates, and also when the boats arrive under the city walls, the same extra - a small, dark woman - can be seen amongst the Vikings in two different places at the same time.
When Princess Gisla touches the flag before the priest blesses it she can be seen wearing very shiny pale nail polish on very manicured nails.
Princess Gisla raises the Oriflamme to inspire the defenders of Paris. However, the Oriflamme was only first used in 1124 by Louis VI, over a hundred years after the events depicted in the series.
King Egbert of Wessex could not have been alive at the same time as Emperor (or even king) Charles in Paris. Egbert died in 839 and Charles did not become king of Francia until 843. He wasn't Holy Roman Emperor until 875.
Gaia Weiss' character is wrongly identified as "Porunn". Actually the name begins with the letter "thorn", a letter common in Old Norse, Anglo-Saxon, Gothic and still today in Icelandic. The name should be written as Þorunn. Obviously, a P is not TH. The same error occurs in the subtitles of the commercially released DVD.