Plot hole: Phoebe and Piper seem surprised at the fact that a white-lighter can impregnate someone, because they are dead. Piper says "how does a white-lighter get somebody knocked up?", but when she went to the future she saw herself with Leo's baby, and Leo told her that he hadn't clipped his wings. (00:46:25)
Continuity mistake: In the church scene when Phoebe and Piper are showing Paige how to use her power, Piper says "according to the prophecy, the third sister has the power to move things with her mind, like Prue could." Prue was not the third sister, but the first. Paige is technically The Fourth Sister, but since the prophecy didn't speak of a "Power of Four", she would automatically become the third sister after Prue's death. (01:02:42)
Continuity mistake: In the scene with Paige and Jordan Bridges' character, nearer the end, when they are sitting on the couch and his possessed character leans in for a kiss, from the back shot, he leans in and his head is tilting towards his right shoulder (you see part of his profile and the back of his head and Paige's face); when they change the shot so you see the front of his face and the back of Paige's head, his head is tilted back towards his left shoulder.
Chosen answer: Her agent/manager negotiated a special billing as part of her contract. It distinguishes her from the rest of the cast, probably when she became a producer on the show.
JC Fernandez