The Courtship of Wyatt's Father - S6-E16
Other mistake: When the Darklighter shoots Leo in the attic on the Ghost Plane, the arrow goes all the way through him. He wasn't shown to be using anything other than a normal Darklighter Crossbow, and there wasn't any evidence that, since Leo and Piper were still alive, that the physics of the Ghost Plane are any different from the Normal Plane. Yet, in every other instance of someone getting shot with an arrow from a crossbow, it never, ever penetrates the person's body, and always get stuck.
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