The Courtship of Wyatt's Father - S6-E16
Revealing mistake: Just after Piper and Leo are taken away, Paige orbs down to the ground and runs up to Pheobe, places her hand on her, and shows a blood-covered hand showing that Pheobe's been injured. But before it's shown, while Paige is running up to Pheobe, you can see Paige deliberately keeping her hand closed and facing away from the camera, because it's already there when she orbs down.
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