When Spike kidnapped Willow and Xander, and wanted Willow to do a love spell for him (to get Druscilla back), at one point he grabbed Willow and threatened her. They have two different angles from which the scene was shot: front and side/back of Willow. When you see her from the back, her hair has been shaken (by being grabbed) and is covering a large portion of her face. When you see her from the front, her hair is all in place. Not a big mix-up until they keep switching angles and you keep seeing the hair go from "perfect" to "disheveled" over and over.