Continuity mistake: When Gandalf, Legolas, Aragorn and Gimli walk into Meduseld, Gandalf is seen arm-in-arm with Legolas, apparently leaning on the Elf for support, but when the camera cuts away and returns, Gandalf is not in contact with Legolas, but they are arm-in-arm again for the next shot.
Deliberate mistake: In the scene after the Helm's Deep battle, when Aragorn meets Eowyn, the earlier footage of Aragorn's return to Helm's Deep is used. He has the wound on his shoulder and is dressed like he was during the 'falling' episode. The background (people in the fortress walking calmly) also indicates that this takes place before the battle. This scene should immediately follow the one where Aragorn is greeted by Gimli ("you are the most reckless man I ever knew"). Confirmed on cast commentary, extended DVD.
Other mistake: At the battle of Helm's Deep, Theoden, Aragorn, Legolas and the rest ride out of the Keep to meet the enemy, watch as the first men out of the gate kill all the Uruks on the bridge, yet the men riding behind them continue to hack and slash away at thin air.
Continuity mistake: When Saruman and Wormtongue look down from Orthanc at the Uruk-hai army, it is spread out across the plain in front of the tower. What happened to the vast caverns which had been dug into the ground? Some are shown off to the left of shot, but the landscape had earlier been shown as much more pitted in other scenes.
Continuity mistake: After Hama says, "By order of Grima Wormtongue," we see Legolas handing a guard his bow, which he was carrying in his hand. Then in the front close-up of Legolas, we see him taking out the twin blades that are on his back and he even does the fancy spin with the blades in this shot. Then there is a shot of Gimli. Then in a side shot of Aragorn, Gandalf and Legolas, we see Legolas again taking out the twin blades that are on his back.
Factual error: When Faramir and his men are looking at the map of Middle-Earth in Henneth Annûn, you can see that the plain of Dagorlad is actually spelled Dagorland on the map. An odd mistake, especially considering the fact that they got it right when they spoke of it in the first movie.
Continuity mistake: During the Warg battle, just after Gimli falls off Arod, the Warg across the field lifts his face and spots Gimli. In the two close-up shots, directly behind the Warg, on his right, there are two arrows sticking up out of a dead body. However, in the wide shot that follows, there is no body with two arrows protruding, anywhere behind him. It is not the bodies to his side.
Revealing mistake: At Helm's Deep, in the 1st close-up, Gimli says, "You could have picked a better spot," and two particular Elves stand at his left. In the next wide shot as Aragorn approaches, the Elves to the left of Gimli are different Elves than in the previous shot. One of those Elves from the 2nd shot is seen close-up when Aragorn walks away. Also, the second row of Elves is much closer to the first row of Elves in the 1st shot.
Continuity mistake: In this shot, Pippin grabs some apples and a green ear of corn, then he puts it all into his left hand, starts to turn around and with his empty right hand he reaches for a floating cauliflower. The camera angle changes, and in the next shot, we see him holding an apple and corn in the right hand and another apple in his left hand. Floating to the right of him is no longer a cauliflower, but a green bottle. Extended DVD.
Deliberate mistake: In the director's commentary, it is explained that the scene of Aragorn arriving at Helm's Deep was filmed early in the shoot, before it was decided that he would have lost the Evenstar. Consequently it had to be painted out. However, when Aragorn turns to leave Gimli a flash of silver from the jewel at his chest still remains.
Continuity mistake: Éowyn and many of the refugees are shown as they see Helm's Deep in the distance. In the first wide shot, Éothain and Freda are riding on the horse and a particular rock formation is seen way in the distance. They keep riding towards Helm's Deep, which we see in the next shot. In the children's next close-up, they are now right in front of that rock formation that was shown in the earlier shot. This is not due to zooming. The two pieces of footage were switched, so as to have their close-up seen last.
Deliberate mistake: During the Warg attack, Aragorn jumps onto a moving Warg and in a close-up, he headbutts the Orc rider. In the next shot, as they're riding, they pass a Rohan rider on a white horse. In the background there are dead bodies strewn on the hill to their right, as they're approaching a huge bush also to their right. As they continue moving, there are multiple shots, including Aragorn falling and stabbing the Orc. In the shot just after the Orc rips off the Evenstar, the Orc falls off the Warg and it is the very same background plate as before, as they pass that huge bush to their right. These two pieces of footage were obviously one sequence that was split and other shots were inserted in between them.
Continuity mistake: Legolas throws down the rope to Aragorn to help he and Gimli escape. Aragorn then grabs the rope with his left hand and holds onto Gimli and his sword with his right hand, plus Gimli's head is lower than Aragorn's. Legolas starts to hoist them up and in the next shot, Aragorn is holding onto the rope with his right hand and Gimli and the sword with his left. In the next shot of them, it's vice versa again. As they're reaching the top, Gimli's head is higher than Aragorn's. Nothing to do with spinning around - just look at the hands.
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.]
Continuity mistake: The bodies and rocks behind Gimli change positions, between shots. When Gimli says the words, "That's not bad for a pointy-eared Elvish Princeling," there is a rock right behind Gimli to his right. In his close-up it's not there. Yet, when the camera does another close-up, when he reacts to the arrow between his legs, the rock is to his right again. When Gimli says the words, "I myself am sitting pretty on forty three," behind him to his right, a Rohan turns over the body of an Uruk onto its back. When Gimli says the words, "He was already dead," that Uruk hasn't been turned over.