Continuity mistake: At the very start of the film when Gandalf is in his wagon, Frodo stops him with: "You're late." Gandalf stops his wagon quite a way from the bank that Frodo appears on. Gandalf sternly delivers his line and moments later they both burst out laughing. Then Frodo almost steps onto the wagon which was much further from the bank during the first part of the exchange.
Continuity mistake: After Sam dives between the Troll's legs and tries to crawl away, in the first shot the Troll lifts his leg and in the second shot he's about to step on Sam, who is in the middle of the floor. Yet in the very next shot Sam is already at the wall raising his left arm and wincing at the giant foot coming down on him.
Deliberate mistake: When Gandalf first sends the Hobbits on their journey, they are in the forest and the Hobbits walk up to Gandalf and the horse, you can tell that the Hobbits are dressed up little people rather than the actors themselves. You can tell by their stature, and how they walk.
Revealing mistake: When the Fellowship is trying to get to the bridge of Khazad-Dum and are delayed by the gap in the staircase, in one shot, Legolas jumps, in the next shot, Gandalf jumps. Sam is visible for a second, in the upper right corner of the screen, and he is obviously the Sean Astin's scale double wearing a mask.
Continuity mistake: When Aragorn tries to save Frodo from the Troll, the Troll throws Aragorn against a wall, and when the camera angle changes to a close-up of Aragorn he is seen lying flat on his back up against a different wall. When he crawls over to Frodo later, the background is the original wall.
Continuity mistake: As Gandalf and Saruman battle each other at Orthanc, both Saruman's and Gandalf's staffs either change position drastically or actually change hands, in consecutive shots. For example after Gandalf 'strikes' Saruman the first time, Saruman slams Gandalf to the floor, Gandalf's staff is in his right hand, yet, in the next overhead shot it's beside him, under his left hand.
Continuity mistake: In Bag End when Bilbo is about to leave, Gandalf leans forwards with his hand on the ceiling or doorway of the circular corridor and says, "Bilbo, the Ring is still in your pocket," then we see Bilbo for ten seconds or so and as he holds the Ring in his hand, Gandalf repeats the motion, in close up.
Continuity mistake: In the scene where Saruman is using his magical powers to bring an avalanche down on Cahadras the group is seen in a close-up to bend over and move against the wall to protect themselves from the falling snow. The camera angle changes to a wide view and the group is seen as just standing there.
Continuity mistake: After Arwen has crossed the Ford of Bruinen and is challenged by the Ringwraith Witch-King, she draws her sword and holds it high and ready to strike. Arwen then calls upon the waters of the river for assistance. When the rush of water with horses' heads comes around the bend and into view there is a clear shot of Arwen's back as she is watching the water come down on the Ringwraiths and she is not holding her sword up. Shortly after all the Ringwraith's are swept away, there is a scene of Arwen's face and upper body and she is still holding the sword high and ready to strike.
Continuity mistake: When the members of the Fellowship are running away from the Balrog, they get caught on the steep narrow stairs and have to jump across the gap. When only Frodo and Aragorn are left on the upper side, there is a long shot from the top, showing large rocks crashing down in the foreground and the two of them further away. Both of them jolt forward, as if with the impact, before the rock has hit the top of the staircase.
Continuity mistake: Boromir approaches the statue that holds the shards of Narsil. First we see the shards laid out on the crinkled grey cloth, with two short ends draped over the edge. Then as he takes the hilt the cloth bunches up (seen on fullscreen), and he cuts his finger on the 'still sharp' blade. In the close-up when he carelessly places the hilt back onto the cloth - as it starts to fall, the cloth (which is now darker) is spread out much straighter, more cloth drapes over the left edge, and the shards are spaced somewhat differently than the previous shot. When Boromir walks away, there is a very short amount of cloth that is draped over the right edge, near the statue's fingers. Then, when Aragorn lifts the hilt off the floor, as he rises, both ends of the cloth that are draped over the edge are now much, much, longer, and the top of the cloth is crinkled, with the shards spaced similarly to the first shot.
Continuity mistake: When Frodo and Gandalf are in the kitchen, and Gandalf is revealing the truth about the One Ring, there's a shot where Frodo is standing, pouring some tea. It then cuts to Gandalf for literally two seconds, and when it cuts back to Frodo, not only has he finished pouring, but he's settled in his chair with his hands wrapped around his teacup. Pretty fast moving there.
Continuity mistake: When the Lady of The Woods meets privately with Frodo, she grabs a pitcher and fills it with water. She is seen holding the handle with her hand about half way down the middle. The camera angle changes and she is now holding the handle at the top of it. The camera angle changes again and she is seen with her hand back in the middle of the handle.
Revealing mistake: In the Prologue, in the shot where Elendil is thrown aside by Sauron and hits a rock and then falls to the ground, you can see his shoulder armor bend, which metal armor should not do. This is because the filmmakers used rubber armor for fighting scenes. This is confirmed on the DVD audio commentary.
Continuity mistake: When Legolas identifies the moving cloud as Crebain from Dunland, everybody hides and when they pass Gandalf is seen standing back up from behind a large rock. The camera angle changes and he is seen again standing up but this time it's from behind a different rock.