Continuity mistake: As Boromir and Aragorn talk right before Boromir dies, you can see Boromir's right hand gripping Aragorn's left shoulder in the shot from behind Aragorn's right shoulder, but when the camera view changes to Boromir's perspective, looking up at Aragorn, his hand is not there. The scene goes back and forth between these two views several times.
Continuity mistake: As Sam, Merry and Pippin run across the grass towards the Bucklebury Ferry, they have two shadows; bold ones behind them, presumably meant to be from the moon, but also unexplained faint ones to their right. (This is before they pass the cottage, so it can't be light from the weak lamps hanging outside.) When Frodo is then pursued by the wraith his shadow is in front of him, yet he is running in the same direction as the other hobbits were.
Revealing mistake: When Frodo stops rolling down the snow covered hill, just before the shot of Boromir picking up the Ring, you can see the edge of the stockings that the actors wore to protect their feet since they were walking in real snow for that scene.
Continuity mistake: The logs that Boromir carries in his arms when he talks to Frodo at Amon Hen are different in pattern and number during the conversation and right before he throws them to the ground.
Continuity mistake: When Frodo tells Sam about the Wood Elves, they both run up the hill and crouch behind a fallen log. In the shot of their backs and the elves in the background, Frodo is on the right side and Sam on the left. In the previous shot of their faces, Frodo was also on the right and Sam on the left. Extended DVD.
Continuity mistake: At the gap in the stairs in Moria, Boromir ducks as he is nearly hit by an arrow, and sheathes his sword. In the following close up he sheathes it again as he stands behind Gandalf.
Continuity mistake: When Frodo is stabbed by the cave Troll, he falls with his face to the left. It remains in this direction as Aragorn crawls towards him, but when he reaches him and turns him over he is facing to the right.
Continuity mistake: After Lurtz releases his first arrow at Boromir, one of the Hobbit stand-ins is seen to Boromir's left, throwing stones. The next shot is of Merry and Pippin standing close together, looking at Boromir to their left - they have moved position to the other side of him.
Continuity mistake: When Boromir attacks him, Frodo frantically slips the Ring onto his middle finger (with the chain). Yet, as Frodo runs up the steps of the ruins, Boromir yells, "Frodo, I'm sorry!" and Frodo wears the Ring on his index finger. He is terrified, so as to remain invisible he does not change fingers.
Continuity mistake: Boromir gets hit by Lurtz's second arrow, drops down to his knees in front of Merry and Pippin, gathers strength, turns around and stabs an Uruk. In the next wide shot and close-up, he stands and is about to stab an Uruk in the back and the two arrows in his chest are there. Yet, in the next wide shot, as his blade is going down into the Uruk, Boromir has only the lower arrow in him, the one near his shoulder is NOT there, for 23 frames. In the next close-up, the upper arrow is back. These arrows are NOT CGI.
Continuity mistake: When Gimli falls onto his knees in front of Balin's Tomb in Moria, in one shot he's a few feet in front of it, in the next he is so close to it, that he could lay his head onto the tomb. In the following shot Gimli is further away again, so he should not have been able to reach it with his head.
Continuity mistake: When the hobbits are hiding under the tree trunk from the Ringwraith in the beginning, you can see space to the left and right of the tree above them. Logically when the Ringwraith walks past the tree you would see it on the right side of the tree first, then on the left, but you don't - it looks like it walks out of the tree instead of behind it. [Confirmed on the commentaries - Elijah Wood asks his fellow actors if anyone spotted the mistake: 'It kind of magically comes out of the tree'. Sean Astin: 'You mean it doesn't pass from the other side?' Wood: 'No, it comes out from the centre.']
Continuity mistake: When the hobbits are sitting at the table in the Prancing Pony in Bree, Merry comes back with a pint. Pippin leaves to get one too. Then we see Sam mention to Frodo that a man is looking at them, across the table sits Pippin and no Merry.
Continuity mistake: Aragorn's mouth is badly bloodied when he is headbutted by Lurtz, but the blood disappears almost immediately.
Revealing mistake: After the battle with the cave Troll, Aragorn rushes over to injured Frodo and brushes past some big rocks that wobble, showing that they are lightweight and not real.
Deliberate mistake: When Sam and Frodo are in the field with the scarecrow, you can plainly see a car cruising past in the distance, from right to left. Further comment - there are two different shots which show the car moving from right to left. One starts at the top right distance, and in a shot a few seconds later the car has traveled down the road a bit and is more easily visible. Complicating matters is that the dust thrown up by the car looks similar to smoke from a chimney in the right distance, making some people think it is just the chimney. But chimneys don't move, and the smoke from the chimney is separate from the moving vehicle. [It is deleted on the DVD, but you can still see an obvious bit of image fakery on the hill just left of the smoking chimney. One can see the hill, tree, and surrounding area move up and down and shimmer slightly where someone has done a cut and paste to cover up the auto. The "car inclusive" scene appears on the National Geographic documentary, "Beyond the Movie The Lord of the Rings: The Fellowship of the Ring." Also, watch the music documentary on the Extended DVD - when it shows this scene the car is still in it. Bizarrely, in his commentary Peter Jackson said he never saw a car and doesn't know what people are talking about, but the production/post-production team say in their commentary that despite not thinking anyone would be able to see it, they took it out anyway.]
Revealing mistake: When Pippin falls in the Midgewater Marshes, you can see that part of his Hobbit foot is ripped off. Extended DVD.
Revealing mistake: Once the secret door to Moria is open, when Gimli says, "Soon Master Elf, you will enjoy the fabled hospitality of the Dwarves," as Gandalf leads the way inside, note the glint of the trailing power cord as it moves along the ground behind Gandalf and goes directly under his robe. This is the staff's electrical supply cable.
Visible crew/equipment: After starting their four day journey through the long dark of Moria, a few shots later Gandalf pulls on his hat brim, and just as he walks (with Legolas close behind) to his left (towards the viewer's right), up some stairs, the black electrical cable leading from the staff to under the robe's left sleeve is visible.
Continuity mistake: As the cave Troll throws Pippin from its back, he is holding his sword in his right hand. When he hits the ground it is in his left.