The Courtship of Wyatt's Father - S6-E16
Continuity mistake: Darklighters are not supposed to die from Darklighter arrows. When Paige and Phoebe get them out of the ghostly plane, a second Darklighter shoots the main one, and he instantly is vanquished. This has never happened to my recollection as DL arrows are meant to only harm Whitelighters or future Whitelighters. (00:35:45)
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