Continuity mistake: When Piper, Paige and Phoebe are having a food fight, Piper freezes the cake, then she throws it at Paige who suddenly has a mysterious stain on her top. Then Piper has orange food on her hands even though she hasn't touched anything orange, and the stain on Paige's top has gone.
Other mistake: During the second attack on Wyatt, when he lets his force field down the top of head is transparent from the special effects.
Visible crew/equipment: In the scene with Chris and David in the attic, a boom mic dips into the shot. (00:28:50)
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