Revealing mistakes: Stilgar supposedly slashes Mohiam's throat, but not only does he clearly not touch her at all, his knife is positioned closer to her chest.
Continuity: In the beginning of Part 2 (The Children), when we first see Leto, his eyes are completely normal. But when the camera turns back to him, his eyes have the bright blue tint from the spice.
Continuity: Jessica's eyes are no longer blue-within-blue, the eyes of the ibad, in Children of Dune, as they were in Dune. Despite the fact she has spent the last several decades on Caladan it is impossible for the eyes to switch back to their normal shade once they have turned.