Goofs

In the middle of their flight, Marcus and Esca lose their beard stubble and are suddenly clean-shaven.

As Marcus and Esca ride across the countryside, they use stirrups, which were not known in Europe until the 7th century AD. This was due to potential insurance issues, especially as Jamie Bell hadn't ridden before this film.

During the final chase scene, it is shown to be raining and yet the dried mud/paint of the Seal people remains pristine and unwashed away. This simply isn't possible.

It's dye. The famous Woad dye of the ancient Britons. It doesn't wash off and prolonged usage ingrains the color into the skin. After a time, it fades, as seen on the Seal People.

When Marcos was honoring the dead body of his father's friend the captain, and set it on fire, a heavy smoke covered almost his face while praying (1:43:16) then when Marcos & Esca put the mascots there were no smoke at all.

That's how lighting a fire works. Lots of smoke until the wood catches then the smoke is gone since it burns.

As Marcus and Esca enter the village of the Seal People, there is a shot of the young boy looking up at them. To the left of him are a pair of legs of a man clearly wearing a pair of modern army boots.

The screams of a red-tailed hawk are used in place of eagle sounds. Red-tailed hawks do not live in Europe.