Continuity mistake: After the potion is spilt, the green stain it leaves on Phoebe's skirt keeps changing both size and colour. (00:31:30 - 00:34:35)
Continuity mistake: After Phoebe tells Piper about the balloon plan, when Piper starts talking about Leo she fills the blender on the counter, and in the two shots facing Piper the white lid is on top of the blender, but in the shot facing Phoebe the lid is gone before she turns it on. (00:19:10)
Continuity mistake: When Leo comes to the manor to tell Piper about Masselin, during the conversation Prue is not wearing her choker in the shots facing her (with her arms crossed) and Phoebe, but in the next shot when Leo tells Piper that he knows she's upset we see Prue's choker suddenly around her neck. (00:11:50)
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