Factual error: On the cliff top before the battle on the beach, you are treated to a lovely if fleeting glimpse of wind farm turbines, not really fitting for the 12th Century.
Factual error: In the beginning of the film, the opening text states that it is the turn of the 12th century. However, the first scene of the movie says that the year is 1199 A.D., which is the turn of the 13th century.
Factual error: The Viking style funeral that Robin's adopted father has would have been unthinkable at the time. Cremation wasn't just against the law, it was completely against the teachings of the Church. He would have been interred in the family crypt or, if that had been destroyed, buried.
Factual error: The French invade the lands of England by crossing the English Channel. The kings of England considered Normandy, Gasconie and Aquitane to be their primary lands up to defeats in the Hundred Years War, about 230 years after the movie takes place. French invasion of England would be illogical and impractical, if not downright suicidal. Not surprisingly, English holdings in France aren't mentioned as a target.
Audio problem: In the first shot of the small church in Nottingham, you can hear the bells ring. However, the bell on top of the church is not moving as it ought to be.
Continuity mistake: On the beach after Robin kills Godfrey, he pulls Marian from the surf to save her and pushes her helmet off into the water, when he's walking up the beach with her in his arms, she has the helmet on again.
Factual error: The invasion was supposed to be at Dymchurch, as mentioned in the Blue Ray director's cut version. In the film, the invasion area is a sandy beach flanked by cliffs. In reality Dymchurch is a pebble beach and is on the edge of the Romney Marsh with no cliffs. The lack of cliffs can be checked via OS Maps of the area.
Revealing mistake: Early in the movie, as they are heading down to the beach in France to board the ship to England, a plane contrail is seen in the sky.
Continuity mistake: The Sheriff asks why Marion is treating him like an enemy when she knows that he wants her, and has just forcefully grabbed her. His right thumb is on her left cheek in medium shots and is missing in close-ups of Marion's face.