Revealing mistake: When Bavmorda and Raziel struggle for control over Sherlindrea's wand (up in the tower Ceremonial Room), lightning bolt shoot out of the wand's tip, and a monolith (among other things) is blasted. The monolith looks like it has been pieced together from cast plastic parts; the fragments' edges are too smooth.
Revealing mistake: Whenever Rool and Franjean have been blue-screened into the movie, the ground does not react to their movements. This is particularly noticeable when they are standing by the base of a tree, surrounded with dirt and pebbles, and when the entire group of brownies are dragging Willow and Meegosh through the dirt.Phaneron
Other mistake: The two headed monster's heads are pretty big. Big enough to swallow a person whole. But when Val Kilmer jumps on one to impale it, the sword comes out of the bottom of the monster's head. For this to happen, the sword would have to be unbelievably large, larger than a man, which he would be unable to wield anyway.