In the episode with the Woogyman, Piper and Prue sneak into the house and they go into the attic to find something in the Book of Shadows. Then Phoebe comes in and locks Piper out. So now only Phoebe and Prue are in the attic. Prue steps in front of the Book of Shadows stand and she talks to Phoebe and then when Phoebe gets ready to swing her knife, Prue (using her power) "pushes" Phoebe (she makes her fly across the room) and it knocks her out. Well later, while they are downstairs, the camera show Phoebe lying down up in the attic and if you look closely, the Book of Shadows stand is closely behind Phoebe, but Prue knocked Phoebe to the opposite side of the room so she should be nowhere near it.