Factual errors: 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.
Factual errors: 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.
Errors in geography: There are no mountains in Denmark. The highest point in Denmark is 147 meters.