When Hrothgar stands with Beowulf and discusses where he got the horn, he makes it sound as if he killed Fafnir the dragon. According to the Viking sagas (Völsunga saga) it was Sigurd (Fafnesbane) who slew the dragon Fafnir, not Hrothgar from the Beowulf epic.
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.
Robin Wright's character tells the King: How can I ever lay with you knowing you laid with her? Of course, it should be "lie with you" and "lay with her" - but then, nobody ever accused Hollywood of being literate or of knowing the distinction between transitive/intransitive verbs.
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.