Continuity mistake: When Beowulf enters the water cave and seeing Grendel's body, the initial shot has his right arm sitting next to him (most likely placed there after his mother's took it from the Meat Hall). When Beowulf leans over to touch Grendel's chest, the arm is gone.
Other mistake: When Beowulf and the Geats discover the cave which hides the entrance to the Sea Hag's cave, there is a light shining up through the pool of water as though it opens up to sunlight on the other side. When Beowulf swims through to the other side however, he emerges into another equally dark cave. A bright clear light is still shining from under the water though. Both sides of the water tunnel open into dark caves. There is no light source, yet the light shines just the same on both sides.
