Revealing mistake: At Rivendell when Frodo puts the Ring on the stone table in the middle and then Gimili says his, "I'd rather die than see the Ring in the hands of an Elf" Gandalf stands up an makes a sort of "you're out- like an umpire" move at someone while they are all arguing, when the camera is on the people. When they go to the reflection on the Ring, Gandalf makes the movement again. Clearly it is the same shot again.
Revealing mistake: After Bilbo returns from the party invisible and finds Gandalf by the fireplace, look at Gandalf as Bilbo walks between him and the fire. There is no shadow cast on him when there clearly should be. Also as the camera pans round following Bilbo's movements, you can see Gandalf is casting a shadow on the floor but not on the table, but when Bilbo walks up to it he does, even though Gandalf is between him and the fire light. (00:25:10)
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. (00:36:10)
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.
Revealing mistake: In Balin's Tomb, after Merry and Pippin are separated from Frodo when the cave Troll smashes its club on the floor beneath them, the Troll first sniffs around the pillar to Frodo's right (viewer's left) as we see Pippin and Merry standing up, before the Troll comes around to Frodo's right and sniffs again. At the start of the next shot, we actually see Merry and Pippin in full view of their facial expressions, standing at the ready, with their daggers held high, continuing to just stare past the pillar behind Frodo. So, despite the fact that the huge Troll is appearing directly in front of them, to Frodo's right once again (before it roars), Merry and Pippin don't even glance in its direction. (00:31:35)