Revealing mistake: Near the beginning of the film, an axe is thrown at a priest. The axe should rotate as it travels, because the head is heavier than the handle. But it hangs upright as if drawn by a wire.
Other mistake: During all of the Battle Scenes there isn't nearly enough blood on the swords. This is most apparent in the last fight.
Continuity mistake: When Merlin is about to strike the ice on the lake with Excalibur he first brings the sword up over his head. When he brings it over his head, his finger is over the hilt of the sword but when he brings it back down and hits the ice with it, his finger isn't. You watch him the whole time so if he moved it you would have noticed.
Continuity mistake: Merlin's hair changes in the last two shots of the movie.