Pre-Witched - S3-E17
Other mistake: During a flash back of Grams in the kitchen she puts a rose in a pot, then Piper walks in, they talk and Piper starts to add some salt and a couple other ingredients into the pot. Grams asks how she knows what to add, Piper says says something about mustard - Piper wouldn't know what's in it already, she just walked in. And she never even finds it strange that there's a rose with a stem in it.
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