Deliberate mistake: In the Chamber of Mazarbul, when Legolas lets loose the double arrows at the Troll near Balin's Tomb, in that shot he stands in front of the same wall as in the next shot. In this second shot he spins around, slices two Orcs and then looks at the Troll in front of him, only this second shot is flipped. In those two shots he stands in front of a solid rock wall with particular markings between two pillars. In the next shot it shows Legolas' back and the Troll down below as Legolas ducks down avoiding the Troll's chain flying overhead. Then in the next consecutive shot from the front again Legolas rises and is now standing in front of a stone wall that has a large alcove with books also between two pillars. These shots were spliced from a longer sequence because the two walls noted are adjacent to each other - in the film the alcove wall is to the left of the solid wall and they share a pillar between them. (00:30:30)
Deliberate mistake: At the beginning of the Fellowship's journey, they are resting on a hilltop (Hollin) with rocky formations. Bill the pony is tethered in the background in one shot. Then come the spying crows from Saruman and they all quickly hide. When the crows swoop in closer we see no-one, but who hid the pony and where? (01:37:45)
Answer: "Fly, you fools"