When Ghost escapes from the Vikings on his first encounter with them, he jumps on a shield and gets hit in the back by an arrow, as he slides down on the shield you can clearly see there's no arrow on his back until in a later shot of the same sequence the arrow gets trimmed by a missed sword-swing from one of the chaser on the "viking snowmobile"
(at around 46 mins) There is a wide shot of Ghost leaving the tribe after warning them about the Vikings being on their way to kill them all, and he is being followed by the mute character. In the first wide shot of the sequence you see them going right through the middle of the screen, the next shot is a close up of the same shot and you can see a trail in the snow ahead of them made by repeating the same shot. The next shot is made in some bushes but the next shot shows the same original wide shot with the same location but this time they are crossing on the bottom right corner, but you can still see the trail in the snow in the middle of the screen from the previous shot.
In the beginning of the movie when Ghost's Indian mother first enters the ship her face is clean, then filthy, then clean then filthy again.
Despite persisting belief, not all Vikings wore helmets and those who did had no horns on them. The filmmakers acknowledged this.
As Ghost leads the Vikings toward the the new native encampment, they pass by a large patch of ferns uncurling their fronds as they mature. With a fairly thickened lake surface just a short distance away, it would be far too cold for the ferns to emerge from hibernation.
After the cracking ice scene, Ghost comes out of the water. In the next scenes he is dry. Wet clothes don't dry during cold weather.
The opening scene has text describing that Vikings were in America 600 years before Columbus. All archaeological evidence and the Viking sagas themselves place them in North America around 1000 A.D. give or take 3 or 4 years, thus only 500 years before Columbus, not 600.
When Ghost splashes into the water after escaping, the metal shield he was sliding on doesn't sink.
An Indian is killed, hanging above the fire, without getting burn marks.
Vikings did not use double-bladed axes or flails.
Vikings did not use plate armour.
At the Spring Trade, white Pekin ducks are waddling around. This variety was not developed at that time and was not introduced to the Americas until 1873.
In the beginning you see a white horse, but horses were not introduced to the America continent until 16th century when the Spanish conquistadors arrive. Also the horse is not of a breed that would have been used by the Vikings.
When Ghost leads the whole troop, most of the time you can hear a wind blowing like a storm, yet the snow falls as it would during a gentle breeze.
At 1:16 into the movie, where Ghost is walking with the Vikings through a camp, an extra on the right side of the screen can be seen hitting his head on a piece of the set, then ducking to walk under it.
The avalanche footage is from a different mountain than the one the Vikings are traveling on.
Neither the mountains nor the forests presented in this film resemble anything found on the eastern seaboard of North America within the last 400 million years.
When Ghost is shown as a child in the flashback, his back is severely cut from his whipping, yet, when the film moves ahead to him as a adult, there is no scarring of any type on his back, yet, the amount of trauma his back suffered would have left some degree of obvious scarring.