Revealing mistake: There are quite a few times where the Uruk-hai fitted skins wrinkle, crease or even bubble up where they shouldn't. Some examples are at Amon Hen, when the Uruk-hai show up and they fight Aragorn, Boromir, Legolas and Gimli. (01:19:25)
Revealing mistake: At the end of the film, there is a shot when Aragorn fights an Uruk-hai, just before Aragorn's back is up against the tree (before Legolas shoots six Uruks in a row). Look at the Uruk-hai's left arm, Aragorn rips two big holes with his fingers, in the Uruk-hai's skin. (01:21:20)
Revealing mistake: While Boromir battles with the Uruk-hai at Amon Hen, to protect Merry and Pippin, there is a close-up of Pippin running towards an Uruk. In the first shot we see Pippin's face. In the next wide shot, not only are the two scale doubles noticeable, the back of the Uruk that Pippin is jumping onto, is also very different than in the next shot. The Uruk's back is much dirtier in this shot, and the clothes are different too. (01:22:10)
Revealing mistake: When Boromir dies, Legolas and Gimli are standing nearby. This shot of Gimli's face and beard are very different here, because they changed Gimli's appearance after this. Most things were shot out of sequence. Compare his face in this shot to when they're fighting the Uruk-hai or when Gimli, Aragorn and Legolas are by the shore and decide to "hunt some Orc." (01:28:10)
Revealing mistake: When Boromir cuts his finger with Elendil's sword, the cut appears before he actually touches the tip of the sword which causes the cut. (01:30:30)
Revealing mistake: As the Uruks run through the woods of Amon Hen, in a close up of a log, the second foot to step on the log is actually a black spray-painted Nike. This is quite visible even without slow-mode, as the Nike symbol is raised on the shoe. (VHS version.)
Revealing mistake: When Gandalf is revealing Bilbo's ring as the One Ring to Frodo, he pulls Sam in through the window and onto a table. When it shows Gandalf looking at Sam, you can see that Sam's hand moves and is positioned in a rather odd way. This is because Sam in this shot is actually the rubber hand of a remote control dummy that they used instead of the actor. In the behind the scenes footage on the DVD you can see other shots of the dummy and Gandalf during this scene.
Revealing mistake: In the first shot of the Fellowship climbing the snow-covered mountain, if you look closely you can see the snow that's in front of them has already been messed-up and trod upon - apparently not the first take.
Revealing mistake: After Boromir is shot twice, he turns around and kills one Uruk Hai, and then when he kills the second one, you can see the sword wiggle after he slices the enemy's legs.
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: 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.
Answer: "Fly, you fools"