Goofs

(at around 1h 45 mins) In the end when Wiglaf is going into the water, the waves break in a pattern that is only present in very shallow water (1-10 cm), but he is in to his waist.

(at around 1h 24 mins) Convex mirrors present a smaller reflection than a flat surface, but when the old Beowulf tells the queen he must fight, there is a well-polished shield hanging on the wall reflecting as a flat surface although it is slightly curved.

Grendel and his mother speak Old English. Even though it is the language of the original Beowulf poem, it is not the language spoken in the time and area where the action takes place. (East Old Norse is more probable.) However, it is "substituting" for Old Norse, just as modern English is "substituting" for the Danes' language.

Though there are no mountains in Denmark (the highest point in modern Denmark being 147 meters) in the age of the Vikings, Norway was also regarded as Denmark, and Norway has many mountains.

(at around 18 mins) "Bards sing of Hrothgar's shame from the frozen north to the shores of Vinland." Vinland wasn't known until about the year AD 1000.

Wasn't known by who? That is a translation of what they called it then in the language they are speaking.

When Beowulf approaches Finn of Frisia, Wiglaf tells Beowulf that the king must not engage in direct battle. Never in any point in medieval Scandinavian history was there ever such a rule...if anything, it was expected that kings led troops in battle.

Initial Correction:

Misunderstanding of the phrase "direct battle". Mano a mano is what's meant here. Not leading troops.

Follow-up Correction

There is no evidence that a king was prohibited from hand-to-hand combat either. King Harold himself was killed in battle at Hastings, as was Harald Hardrada at Stamford Bridge and many much earlier.

One of Beowulf's men sings about a whore from Norway. It is unlikely that "Norway" would be used as a place name before the unification of Norway in AD 872, almost four centuries after the scene's setting.

Also unlikely they'd be singing in contemporary English. That is a rough translation of what they were singing.

Grendel and his mother say several phrases in Old English. Even though it is the language of the written version of the Beowulf poem, it is not the language spoken in the time and area where the action takes place (East Old Norse is more probable). There is also no justification for switching from Modern to Old English and back in the dialog.

Historical nitpickery. They need no justification for speaking in whatever language they want.

The movie depicts Christianity displacing the Old Norse religion from Denmark over the 6th century AD. In reality, Denmark did not become Christian until the 9th century.

The seeds were sown long before that. It wasn't like flipping a switch.

(at around 5 mins) In the movie, a black rat is seen, but the black rat first appeared in Denmark in the 12th century AD.

The singer in Beowulf's retinue sings about a whore from Iceland, but Iceland had no permanent population before AD 870-874, and certainly was an unknown land with no human presence at all in AD 507.

Unknown to who? That is a translation of what they called it then, in their language.

(00:59:15) Grendel's mother is walking on the water in spike heels.