Hrothgar's hall Heorot is depicted as a Roman villa in painstaking detail, even though the 6th-century Hrothgar would most likely have had little to no contact with the remnants of Roman civilization.
Scandinavian warriors did not wear horns on their helmets. Given the close-quarters nature of many Scandinavian combats, this would have been foolish.
Although supposedly Vikings, almost none of the weapons are in any way similar to those used by Vikings. Beowulf's sword is pure fantasy, as is the scope equipped crossbow. Other swords, pole arms, and shields are from centuries later.
The Vikings had no knowledge of explosives as used with Beowulf's crossbow.
An obvious CG effect is used to depict Grendel ripping the guts out of one of Beowulf's band - the wound shifts position on his stomach and disappears when he falls.
During the fight with Grendel at Heorot, in some shots Beowulf's helmet is missing a horn.
Much of the armor worn was of a design that was current many centuries after the poem was actually set.
Unferth watches Beowulf's crew land on shore using a brass spyglass of 1800s' vintage.
The village of Horsens (today the town) didn't exist before at least the 10th century.
The ship that Beowulf and his company take sail on is a caravel, a type of ship which would not appear for many centuries.
Stirrups are seen on the horses. These were not introduced into western Europe until a number of centuries later.
Heorot is shown situated high up in an alpine setting with a backdrop of rugged mountain peaks, however Denmark is a rather flat country with no significant mountains.
Why did Beowulf carry his young companion all the way back to Heorot when it was established a minute later that he was perfectly capable of walking?