During Beowulf's story of his adventures, he makes reference to how he "beheaded Grendel's mother in the hall with a mighty sword." (2139-2140) This seems to be in contradiction to the original account of the action in an earlier passage. When we first read about the battle, the text tells us that Beowulf beheads Grendel, not his mother. This happens just after Beowulf defeats Grendel's mother in her underwater lair:

Beowulf in his fury
now settled that score: he saw the monster
in his resting place, war-weary and wrecked,
a lifeless corpse, a casualty
of the battle in Heorot. The body gaped
at the stroke dealt to it after death:
Beowulf cut the corpse's head off. (1584-1590)

Perhaps the contradiction is due to a mistaken translation of the passage. It is possible that the translator could have misinterpreted the original Old English text when translating it.