Other mistake: John Noble's name is misspelled in its second appearance in the SEV credits. It is correct in its initial appearance (the fades with the fancy font). However, in the short list of where the actors are listed with their character's name, it is spelled "John Nogle."
Deliberate mistake: When Aragorn, Gimli and Legolas stand and then enter Fangorn, there are multiple shots that are flipped and then not, back and forth. For example, when they get ready to fight the White Wizard, the shots are flipped. The quivers and brooches of Aragorn and Legolas, are the wrong way.
Continuity mistake: While running in Fangorn Forest, Aragorn stops kneels down and then says, "These are strange tracks." In the wide shot preceeding just as he kneels down, and in the close-up when he says those words, his position and the position of the rocks and trees surrounding him are very different in both shots. This has nothing to do with camera angle.
Continuity mistake: When the Uruk-hai are sending the three huge ladders up on to the Keep, watch the last one just before Legolas cuts the rope that is pulling it up. The ladder has just passed the 90 degree/vertical point. Naturally, after that it starts gaining speed as it falls. Then it cuts to Legolas, who waits a second or two before firing his arrow. That would be plenty of time for the ladder to pass the point of no return, and hit the wall, so it would not have mattered if Legolas had shot it or not it would still have reached the Keep.
Continuity mistake: At Henneth Annun, Frodo and Sam have their blindfolds removed, and to their left are two barrels. In the 1st shot, one barrel has a brown cloth draped on it with a bowl of nuts and cracked shells and on the second barrel, candles that are not lit. In the 2nd shot, Faramir walks in and to his right the brown cloth under the bowl of nuts is draped differently, the candles, which are lit now, are a different variety and there are also ceramic jugs near the barrels now. After Frodo answers, "Yes. For my part," Faramir gets up and as he walks, the items on the barrels look the same as in the first shot.
Audio problem: First there is a wide shot of the Dead Marshes and Gollum says, "Soft and quick as shadows we must be." Then in the next shot as the camera starts to pan towards Sam, he begins to say, "I hate this place, it's too quiet..." When the camera first shows Sam, his mouth is not in sync with these particular words, "I hate this place," the words originally said on camera were changed during ADR. Extended DVD.
Continuity mistake: At Edoras, after Wormtongue spits on Aragorn's hand and runs away, a man shouts, "Hail Theoden King!" The crowd bowing at the bottom of the stairs in this wide shot, is more dense, than in the close-up shot after, where the crowd is very sparse to the right. Not to mention some people are arranged quite differently.
Other mistake: When Aragorn & Gimli are being pulled up the wall after defending the gate at Helms Deep, Gimli is a puppet (A very good puppet though.) when you see him in Aragorns arms just before they get pulled over the top by Legolas, only a quick shot but still visible.
Revealing mistake: After Haldir dies, Aragorn jumps onto the only ladder resting against the steps 'inside' the Deeping Wall. Hanging onto the ladder, he then falls on top of the Uruk-hai down below. The ladder is now lying on the ground perpendicular to the steps of the wall. However, when Aragorn runs up the stone steps on the side of the Keep, in the aerial CG shot of the wall below, the ladder is not lying on the ground where it should be, under the scurrying feet of the Uruk-hai.
Continuity mistake: At the beginning of the scene, Sam is cooking the rabbits. Behind Gollum, Frodo's coat and pack lay on the left side of the overturned stone carving. In the following close-up, when Frodo goes over to his pack to take out plates and spoons, his coat is now on the right of the overturned carving. In the next wide shot, it's on the left again.
Revealing mistake: Right after Gandalf gave warning that they would not be welcome at Rohan and jumped forward with his horse, next after him Legolas moved forward before Aragorn, you can see Gimli that rode with Legolas was a fake Gimli, the color of his skin was not normal, and he was too stiff.
Continuity mistake: When the Uruk-hai are being attacked by The Riders of Rohan you see Pippin on the floor with some mud on his face, yet in the next shot when he is almost crushed by a rearing horse he has got no mud on his face.
Continuity mistake: As the Uruk-hai charge the walls of Helm's Deep, the elves release their arrows first, no problem here. However, when the Rohirrim shoot, the arrows that strike the enemy are not the short, brown-leather fletched arrows that they should be using, but the long white-fletched elven arrows.
Continuity mistake: When Gollum and Sam and Frodo are by the river and Sam and Frodo are talking about Gollum. Frodo turns around and yells something like, "What do you know about it?" When he yells this, he and Sam's faces are a few inches apart. When it cuts to both of them, they are about a foot apart.
Revealing mistake: Just as Theoden yells, "They have broken through, the castle is breached." Legolas is first seen on the exterior wall pulling the string of his bow with NO arrow in his hand twice, but the other times he pulls the string on the bow we do see the arrow before reaching the bridge. Also, after he crosses the bridge, before he gets to the stairs, when he reaches behind him for an arrow the arrow is on his back, not in the quiver, it's just simply on his back. (Do slo-mo or frame by frame if necessary).
Deliberate mistake: During the Warg attack, Aragorn is being dragged by the Warg and is heading for the cliff. In the very last wide shot of Aragorn, from behind his head, we see the hillside and riders behind the Warg, where they just came from. Next, we see a couple of shots of Gimli finishing off a Warg. In the next wide shot, the hillside including riders and bodies is the same one as earlier, only it's been flipped.