Other mistake: Just as Gandalf and the Rohirrim are riding into the Uruk-hai and they clash, go slowly, frame by frame. Legolas will be on the right of the screen. Keep going frame by frame. You'll soon see a few pretty clear frames of Arwen with her sword, on her white horse. She's sitting to the right of Legolas. Her long hair is pulled back into a braid. She's wearing pink sleeves. She's the only one on the screen with that color, so you can't miss it (slow motion can be helpful but is unnecessary). The official reason for editing out Arwen (given on the DVD extras) was that during filming Peter Jackson intentionally leaked an image of Arwen at Helm's Deep online to gauge fan reactions, then backpedalled over her presence after overwhelmingly negative fan reactions. (01:30:40)
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. (01:04:50)
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. (02:38:30)
Other mistake: In this scene, Legolas says, "The Uruks turn Northeast. They're taking the Hobbits to Isengard." This is a mistake because Aragorn, Gimli, and Legolas are chasing the Uruks on a westerly course towards Isengard. If the Uruks had turned Northeast, they would have almost doubled back on the trio. (This is in the "Special Extended Edition" DVD.) The Uruks were taking the closest way to Isengard, across terrain, not traveling by common roads. In the book they traveled without rest. There's no reason to believe they would have diverted from their Western course to Northeast temporarily, that would have slowed them down. (00:19:00)
Other mistake: When Aragorn, Legolas, Gimli, and Gandalf are at Edoras and are first entering Theoden's hall, watch the right side of the screen as Wormtongue says, "I told you to take the wizard's staff..." Legolas delivers an uppercut to a guard, but his arm goes way above his head. Furthermore, the punching sound effect is not heard until his arm is way over his head. Legolas is as tall as the guards and his fist would have stopped as soon as it hit the guard's face. There is no way the guard can be that tall. (01:19:05)
Other mistake: Before the battle of Helm's Deep, all of the Elves are positioned along the top of the wall. Their waists are right up against the shorter wall to protect them somewhat from incoming crossbow bolts. The Uruk-hai are heard arriving from a distance and Aragorn walks out a few feet in front of them to survey the battlefield. How could he walk out in front of them when they were positioned right up against the edge of the shorter wall?