Continuity mistake: When Prue talks to Piper after stopping her phone conversation, she tells her that "the Sin Balls work a lot faster than [they] thought. Once infected, the victim only has a few hours before it drives them to total self-destruction." However, this is almost verbatim what was written in the Book of Shadows. It could be easily read by the viewing audience when the book entry was shown. So all three sisters should already know this.
Continuity mistake: When Prue telekinetically moves the sin ball box across the street she moves it ALL the way across but when the man infected with greed goes to get the box, the box is only halfway across the street.
Continuity mistake: When Prue and Piper leave to help the hostage, Piper asks Leo, "Do you think she's mad?" Her hair is in front of her shoulders. Immediately afterwards, she sits on the couch with Leo. Her hair is behind her shoulders.
Continuity mistake: During the scene where Piper is ordering merchandise from Bloomingdale's and Prue, she and Phoebe realize the "sin balls" infected them. Piper's hair constantly changes from being in front of her shoulders and behind them.
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