When the priest is on his knees praying at night, Grendel steps up behind him twice (from two different angles).
When the priest is talking to Beowulf on the morning after his arrival in Daneland, his staff goes from being clenched in his hands, to resting against his shoulder in between shots.
At the beginning, when Grendel's father is running with his son in his arms, it is clearly a dummy.
Brendan is referred to several times as a Celt, but the word was not applied to the inhabitants of the British Isles until the eighteenth century, and would not have been used at the time.
At 1:30:25, when Beowulf enters the cave on the beach you can see an orange extension cord in the bottom right corner of the screen.
While the Daneland portrayed in the movie has many mountains, cliffs and rocks, the real Denmark does not. Denmark has no rock formations, and very few steep cliffs.