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).
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.
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.
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.
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: After Frodo wakes up in Minis Tirith, Merry and Pippin stand in the doorway. Their height reaches the middle of the ornate carving on the door in the close-up, but 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.
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.
Trivia: When Gandalf is talking about the gathering of the armies of Sauron, the next shot shows the Corsairs on a ship. Walking there from right to left is Peter Jackson in a cameo as a Corsair pirate. In the Extended DVD, at the start of Disc 2, he is actually pierced in the chest by Legolas' arrow and dramatically dies!
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.
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: Smeagol walks into the cave as his voice-over says, "They called us murderer." He coughs, and "gollum, gollum" comes out. In the 1st wide shot as he's clutching himself, Smeagol's fingernails are very thick, long and stubby. In the 2nd, a close-up of his hand, it reveals the Ring in his palm and his fingernails are perfectly normal and flat with dirt underneath. In the 3rd, a wide shot, his hand is open and the fingernails are very thick, long and stubby again. The 1st and 3rd shots are one sequence that was split to add the close-up in the middle.
Continuity mistake: When Faramir is carried up and placed near the White Tree, Denethor walks down the stairs to approach him. The two soldiers who stand on the stairs outside the palace doors repeatedly change from standing on the 4th step, to the 5th step, then to the 4th, back on the 5th, and so on, in the first and all the following shots.
Revealing mistake: Trying to save Faramir, Pippin shouts, "Gandalf!" and in the first shot from behind Pip, he runs towards Gandalf, the Gondorian soldiers run towards the camera and he is perfectly Hobbit sized. However, in the next shot, facing Pippin, he is not only further away than in the previous shot, when he runs towards the camera as the Gondorians run past him, Pippin is now much taller, almost as tall as the soldiers he passes.
Continuity mistake: When Gollum accuses Sam of eating the lembas bread, the crumbs on his shoulder don't appear until the shot when Gollum brushes them away.
Continuity mistake: Just before and after Sam yells, "Frodo get down. Hide!" the sores at Frodo's neck caused by the Ring's burden are visible. Frodo then falls to the ground after seeing the Eye and the sore on the right side of his neck is seen again, but completely different. The sores change many times in consecutive shots.