The saber Ibn Fahdlan makes is a curved sword which is exclusively Turkish. Arabs at that time used straight swords similar to western broadswords. An Arab nobleman wouldn't know how to make one even if he had an anachronistic/exotic taste for Turkish cavalry sabers. Turkish style curved swords became fashionable in the Middle-East after large groups of Central Asian Turks began to be enlisted by Islamic armies later.
At the beginning of the movie, Ibn is in love. He encounters a woman in a corridor, who is wearing a strange headscarf over the low part of her face. Not only is this clothing not opaque, which is forbidden to Muslim women wearing headscarfs, but it is a Yashmak, worn first by Turkish women around 1840.
Helfdane (Clive Russel) and Buliwyf (Vladimir Kulich) wear Armour that was not invented until centuries after the film is set. Helfdane wears a solid breast breastplate and a helmet, the latter is a Spanish "Morion" helmet that did not appear until the 16th century.
Buliwyf wears a set of plate Armour, a type which was not developed and used until the 15th, early 16th century. The movie takes place in the 10th century.
When Buliwyf, in the scene where the 13 are selected says: "I will be the first man", he is speaking in a strange Scandinavian dialect, saying: "Jeg er være første mand" (written in Danish), meaning: "I am will be the first man".
In the scene where the warriors are outside talking in a circle (just after they have met with the king), you can see one of the warriors with a war hammer resting across his shoulders. The scene shows the hammer end up and the spike end down, but in the next shot the ends are reversed, and then back again.
After one of the warriors claims "only an Arab would bring a dog to war," Ahmed displays his horse's abilities. One shot of Ahmed on his horse is shown in the rain immediately before hurdling one of the horse-mounted warriors. It is not raining during the rest of the scene.
When Herger takes Angus' head, Angus is facing Buliwyf and a waist-high rock, but when Herger walks away and Angus falls, Angus has turned 180 degrees and is facing away from Buliwyf and towards a much taller rock.
During the Angel of Death's "recruitment drive", and just after she announces the 13th warrior must not be a Northman, Herger is standing up and looking at Ahmed Ibn Fahdlan. Just few seconds later, Herger is seen sitting down with a different set of clothing.
Ahmed Ibn Fahdlan Ibn Al Abbas Ibn Rashid Ibn Hamad after receiving the chain mail is seen struggling to put on the top on his way to his post on the barricade then we see him in full chain mail climbing the ladder.
One warrior is missing.
Two die in the first fight.
Four die in the first battle.
There are seven left. This can be seen when they are riding for the cave. However only six enter the cave. One was lost.
At one point, Ahmed grinds a straight broad sword into a curved scimitar. First, such an alteration to the blade could not be done with a grindstone. Such a task would require heating the blade and pounding it into the proper shape. The grindstone would only be used to polish and sharpen the blade. Also, this kind of alteration without heat treating and tempering the blade would compromise the integrity of the steel, resulting in a weapon too fragile for use. Finally, it appears as though it takes Ahmed, a man untrained in smithing, only a few hours to make these modifications on the grindstone. In reality, it would likely take a trained swordsmith days, if not weeks, to complete such a job.
Melchisidek speaks Greek. The DVD subtitles correctly say "[speaking Greek]" while he does. The Northmen recognize but cannot speak Greek. But Herger can speak Latin and replies in that language, the other "learned" language of that era. Melchisidek can speak Latin and shifts to that language. However, the DVD subtitles do continue to incorrectly say "[speaking Greek]" after the change to Latin.
While most warriors discarded their original weapons to swim under the cliffs, they acquired new ones: Buliwyf is given a sword by the king, Herger uses an unfamiliar halberd, and Edgtho's knives are borrowed. Thus, the last fight is not wrong.
Right after Herger says to Ahmed Ibn Fahdlan, "Relax, little brother, there are more", Ibn runs his sword through a monster. The sword seems to be sticking out of the monster's back but if you look closely you'll see that he is only holding the sword under his armpit.
When Buliwyf (Vladimir Kulich) is seen using his double-handed longsword to kill the bodyguard of the "mother" in the caves, the sword is obviously a lot shorter than seen earlier for practical reasons. Had the sword been original length the killing of the bodyguard would not have worked as depicted.
After the first armed skirmish in the throne room, Roneth the Horseman, finds the headless body of Ragnar the Dour. Buliwyf asks, "What about Olaf?" Roneth uses his torch to shine light on the body of Olaf (without his head) and says, "It's the same," (confirming the Wendol always take the heads of their victims). The body of "Olaf" is actually that of Hyglak the Quarrelsome, easily identifiable by his prosthesis Axe-hand. It is possible "Olaf" was a second name of Hyglak. It is also possible that due to the numerous movie delays, director changes and script rewrites, "Olaf" was the final name of the character and "Hyglak" an older version that did not make it into the final movie but was still spread to the press.