Continuity mistake: When Denethor holds the Horn of Gondor, the metal loop (with rope), which is affixed at its wide end, is located at the outer curve where it is cloven. However in both FotR and TTT, the metal loop at the Horn's wide end, is located on the inner curve where it is cloven, on the opposite side!! (There are only two metal loops and they are permanently affixed.)
Continuity mistake: In the second half of the film, Frodo has a scar on his lower right cheek, close to his chin. Many times throughout the rest of the film the scar changes position and size on his right cheek. It also appears on his left cheek in flipped shots (most obviously on the slopes of Mount Doom when Sam is cradling his head).
Continuity mistake: As Sam lay sleeping, Gollum sprinkles lembas crumbs on Sam's right side. 1st shot, the grey blanket completely covers Sam as Gollum begins to sprinkle it. 2nd shot, the blanket is at Sam's shoulder with a just a bit of cloak and sleeve showing. 3rd shot, the blanket is completely covering Sam's shoulders as the lembas is shown only on the blanket. Then Sam stands and the blanket is now on the ground (as mentioned in another mistake, there are no crumbs at all when he stands). Later, when Gollum accuses Sam in the 'framing' shot, the lembas crumbs are shown on Sam's cloak even though it was the grey blanket Gollum sprinkled it on earlier.
Visible crew/equipment: At the Black Gate, Sauron's forces march towards Aragorn and his army, there are close-ups of Pippin drawing his sword and of Merry taking a deep breath. The next shot is of Orcs running to the right of the screen. As that shot opens, click five-six frames in and TWO crew members are seen between the Orcs on the right of the screen, click a few more times and more crew members, one with a hat, are visible in the middle, between the Orcs.
Revealing mistake: After the Witch King destroys Gandalf's staff, the wizard and Pippin fall to the ground in the wideshot. In the next close-up, as Pippin (who is behind Gandalf) raises his right leg while he gets up, the white/black sneaker he wears is visible on Billy Boyd's Hobbit foot.
Visible crew/equipment: In Shelob's lair, when Frodo is caught in the web, in the first close-up while hanging there the camera pans down to his feet. Both ankles are wrapped and have foot supports and the wire cables attached to his ankle straps are visible, especially his left. In the next back shot, as Frodo turns around the wrist/arm support at his right arm, while he holds Sting, is seen too.
Visible crew/equipment: When Sauron's forces start to surround Aragorn et al. at the Black Gate, there is a close-up of Éomer glancing towards the right of the screen. The next shot is of Orcs advancing to left of the screen. Fourth frame in, a dark haired crew member is visible behind the Orcs near the center, click some more and again he and others are visible in the background.
Continuity mistake: When Frodo is stabbed by Shelob (the giant spider) and wrapped in her web, Sam thinks he's dead because he's not moving at all. From Sam's point of view Frodo's eyes are fixed wide open. A cut to a side view of Sam lifting Frodo shows Frodo's eyes shut. The next cut back to the original point of view shows Frodo's eyes back fixed open.
Continuity mistake: After Frodo wakes up in Rivendell, Merry and Pippin stand in the doorway. Their height reaches the middle of the ornate carving on the door in the close-up, yet in their next wide shot running in, they are considerably lower. When Legolas and Aragorn walk in, also in a close-up, their height is just a bit higher on that ornate carving than the two Hobbits' heights were. Then when Sam stands in the doorway his height also reaches the center of the carving.
Continuity mistake: After Gollum says, "Clever Hobbits, to climb so high!" he jumps onto Frodo and Sam. Gollum then attacks Frodo and when he turns Frodo onto his back, Frodo's shirt is wide open at his neck/chest and the necklace and ring are gone. In the next close-up as Gollum chokes Frodo, and the following shots, the necklace is back.
Visible crew/equipment: As Sam carries Frodo on his shoulders, Gollum jumps onto them. When Gollum and Frodo fall down the stairs and then Frodo lies on his back, the black body protector that Frodo wears is first visible at the neck and then under Frodo's front shirt flap that is untucked.
Continuity mistake: When Theoden is dying, the Witch King says, "Feast on his flesh," and there is no Mumak behind Theoden. As soon as Eowyn appears there is a giant Mumak carcass lying behind Theoden. The beast is seen in a few shots, but in quite a few shots, even the birds-eye views, no Mumak is anywhere near Theoden.
Continuity mistake: In the opening shot as Gandalf stands on the balcony at Minas Tirith the many buildings beside the balcony are shown. When Gandalf says, "If the garrison at Osgiliath falls, the last..." as he continues, "...defense of this city will be gone," the buildings beside the balcony are once again visible. To say that the buildings are completely different in the two shots would be a vast understatement! (This has absolutely nothing to do with camera angles. Extended Edition.)
Continuity mistake: At the end, when the four Hobbits are entering Grey Havens, Sam is wearing a yellow vest with brown buttons over a white shirt. The vest can still be seen when Sam and Frodo hug. But when Frodo makes his way onto the ship and turns back to smile at his friends, Sam is no longer wearing the yellow vest. The vest is still not there when the three Hobbits turn to leave Grey Havens. But, when Sam returns home to the Shire and is holding his daughter, he is once again wearing the yellow vest.
Visible crew/equipment: While Frodo is tied up in the Tower of Cirith Ungol, Gorbag says, "It's going...along with everything else." In the next shot of Frodo, behind his right shoulder a crew member is moving around in the background. It is not Shagrat or Gorbag. Also, when Gorbag says, "...and I'll stick this blade in your gut," in the close-up of Frodo as he moves his tied wrists, between his right elbow and waistline of his pants a crew member's face is visible at the top left of the screen. It is not Shagrat or Gorbag. (Can easily be seen slow motion or frame by frame.)
Continuity mistake: Gollum says, "Clever Hobbits, to climb so high!" and leaps onto Frodo and Sam. When he rolls down the steps with Frodo, while Frodo is lying face down, one of Frodo's back suspenders pops off and the button can be seen flying off his back towards the ground. Of course the suspenders are fine in the following shots.
Continuity mistake: In one of the final scenes of the movie, Frodo is writing in the book "There and Back Again," adding his own story. As he is finishing, he clutches the wound he received from one of the Nazgul in "The Fellowship of the Ring." Frodo does put the quill down, but that happens before he gets the pain in his chest. Frodo has the quill in his hand when he starts to write again. Frodo is not shown putting the quill down between these next consecutive shots. In the first close-up, Frodo has the quill in his hand as he clutches his chest, because of the pain. Next shot, Sam enters the room. In the next consecutive shot, Frodo, who is still clutching the pain in his chest, does not have the quill in his hand.
Deliberate mistake: In the Extended Edition, the location of the Tomb of the Stewards is shown to be behind the city of Minis Tirith, at the cemetery on the mountainside, accessible via a long bridge. Flaming Denethor later runs to the far prow of the city, leaps off and plummets. The distance between the two areas makes it is obviously entirely impossible for someone, even adrenaline charged Denethor, to run this distance while his oiled flesh burns. Admitted on the commentary that it's poetic license, due to director Peter Jackson's long awaited vision for this dramatic shot.