Factual error: In the scene, Floki walks towards a small waterfall today known as Kvernufoss. Floki is supposed to be the only inhabitant if not the entire Iceland, at least the region, as we are shown no sign of civilisation whatsoever, and as historical record tell us about Iceland in the early 800s (as the events of Vikings seem to take place). But a well beaten footpath is visible, leading to the waterfall at the end of the canyon. That path is an oft-travelled path tourists follow nowadays to reach Kvernufoss, a well-known site in the area. A path like that in Floki's time would simply not have existed (there is nothing in that canyon but that waterfall, no reason to go there if not for sightseeing) and represents a (tiny) intrusion of our modern world in a show set in the middle age Iceland. (00:37:42)
The Message - S5-E7
Factual error: When Floki lands in Iceland with the other Vikings brought from Kattegat, they land on the beach near Dyrhólaey, the southernmost tip of Iceland. Moments later, they are shown walking near the waterfall of Dettifoss, on the opposing side of Iceland. That is almost 300km (186mi) away, assuming they walked on a straight line across mountains, glaciers, valleys and a highly inhospitable desert in interior Iceland. If they went along the coast, that's more like 625km (390mi). (00:30:55 - 00:31:16)
The Message - S5-E7
Revealing mistake: When Floki and fellow Vikings march across Iceland, an aerial view of Dettifoss waterfall is shown, including the visible tourist paths, fencing and a small stone bridge on the left, used by tourists to visit around the site. (00:31:15)
Revealing mistake: The battle has Lagertha fighting skeletons, but on her final skeleton she swings her sword downwards and massively misses it by a large margin, but it still falls.