Continuity mistake: In Faramir's hideout behind the waterfall, Sam tells Frodo, "Use the Ring, Mr Frodo, just this once..." When the camera is on Frodo alone, the camera lighting shines on Frodo's right side, with his left in shadow. When the angle of the camera changes to show both he and Sam, Frodo's left side is then lit.
Other mistake: When the men are retreating into the halls of Helm's Deep, you can see an archer in the bottom right corner repeatedly shooting his bow without an arrow on the string.
Other mistake: It seems as though the Elves at the battle of Helm's Deep aren't quite as coordinated and elegant as Elves are rumoured to be. During the Helm's Deep battle scene, Aragorn is shouting commands to the Elven archers. In the close up shot of Aragorn while it's still raining, look to the left of Aragorn, there is an Elf who gets hit in the head, by the quivers, on the back of the Elf next to him.
Continuity mistake: When Faramir and his Lieutenant are looking at the map, just after it's been unfolded, the Lieutenant puts his right hand out and points on the map with his right hand. In the following close-up, his left hand is doing the pointing on the map. All the close-up shots of the map were added later during pickups, to show more clearly where various characters are.
Continuity mistake: At the end of the film, when the heroes of the big battle are watching the lightning over Mordor, while Gandalf says the words, "The battle for Helm's Deep is over", over Legolas' right shoulder is a far view of the Deeping Wall and the Causeway. There is no break in the wall, or ladders at the wall, nor are there any bodies anywhere on the Causeway or otherwise.
Continuity mistake: When Gamling is fighting the Warg scout, after Hama's death, Legolas jumps off the high rocks, and starts running towards the Orc. There are high rock ridges behind and to the left of Legolas. As soon as the Orc scout falls to the ground, in the next shot of Legolas, as he runs toward the Orc, that background is flat and there is only one rock ridge directly behind Legolas.
Deliberate mistake: During Theoden's attack, Arwen on a white horse is visible in about 3 frames. It was confirmed on the Commentary that Arwen took part in the Helm's Deep scenes as originally filmed, but was cut from them later, but apparently not completely, because she is seen in a number of shots. During the beginning of the attack, when the riders are still in the fortress, several shots end abruptly when only the white horse (different from Arod, its head is not covered with protection) is visible and not the rider. She is also seen riding down the causeway, in front of Legolas. Her hair was colored blonde, her body and clothing were shaded and colored green, to disguise her.
Continuity mistake: At Edoras, as Éowyn says, "Now the Wildmen are moving through the Westfold, burning as they go, rick, cot and tree," Aragorn and Gimli sit at a table across the room. In this shot, the candle on their table has drippings going down the side of the candle. When Aragorn says, "You have two thousand good men..." the candle is taller and does not have those drippings. Aragorn says, "Open war is upon you..." and the candle now has more drippings than in that first shot.
Revealing mistake: When Theoden says, "They will break upon this fortress like water on rock," the camera is in a crane shot which slowly creeps up, becoming a medium close-up of Aragorn. As the camera flies over the front wall of Helm's Deep in the shot where Theoden delivers his line, look down into the deepest level of the castle towards the left of the screen. Rohan citizens are bustling and moving. If you watch closely, you can see the lowest level of the castle jump and shake in the shot. This is because that lowest level wasn't there in reality and it was added in the computer. (Probably can be seen only in the Widescreen version).
Continuity mistake: When the Berserker Uruk-hai with the torch runs to the bomb to detonate it, Aragorn yells at Legolas to stop him. Legolas fires an arrow with green short fletch (feathers), then you see the berserker hit by a long white fletched arrow, the kind the Lothlorien Elves use. A second time, Aragorn yells to Legolas to kill him, and then again, the arrow shot by Legolas is not the same type as the one that hits the Uruk-hai. The Uruk is NOT hit by arrows of another Elf, it is Legolas that hits him, twice. [People keep trying to correct this by saying that Legolas reuses arrows, but read the mistake properly. We SEE the arrow he fires, and we SEE that the arrow which hits isn't the same as the one he fired.]
Visible crew/equipment: When the wounded Aragorn gets up on Brego the next shot shows him ride towards the sun, and his shadow behind him. The same thing goes for the camera crews shadow (you may have to play it in slow motion in order to see the large shadow). The shadow of the crew is seen at the very beginning of this shot. And there are no rocks beside the camera because the camera moves 180 degrees to show open terrain.
Deliberate mistake: As Aragorn, Legolas and Gimli gear up for the battle of Helm's Deep in the Keep, Gimli puts on chainmail. After he says it's too tight in the chest, you see a back shot of Gimli standing facing Aragorn and Legolas. This man is obviously a scale double, as the body and posture are wrong.