Coyote Piper - S3-E9
Other mistake: At the beginning of this episode, Tara and Kierken are arguing and Kierken says that he could always "suck you into your little mixing bottle and start again." Then Tara says that "she has a better idea," and then she stabs herself. The life essence automatically flows out of her but the knife was still in her body. Later, the possessed business man stabs himself in front of Piper and she kneels down to see if he was okay and the essence flew into her, without the knife being removed from his body. At the end, when Leo comes down after Prue stabs the possessed Piper, he kneels over her, but the essence does not flow into him. It only flows into him after he takes the knife out of her body.
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