Revealing mistake: When Phoebe goes to sign her papers at the end, she spells her name as Pheobe. (01:22:45)
Siren Song - S5-E4
Revealing mistake: When the Siren is being healed, you can see that she's just holding the pole next to her, when it's supposed to be going straight through her. (00:19:50)
The Importance of Being Phoebe - S5-E11
Revealing mistake: When Kaya throws the knife at Phoebe watch in freeze-frame and you can see that for the shot from behind her there's no knife in her hands. (00:36:10)
Revealing mistake: The character Ramus (the elder) changes actor when Leo jumps and orbs to get him away from the attack of the Aggressor. The actor's stand-in is visible instead. (00:18:52)
Revealing mistake: When Piper blows up the painting you can see the hole used to put the charge into to blow it up. (00:57:40)
House Call - S5-E13
Revealing mistake: When Piper made the house vanish, and she, Paige and Leo ran up to where it was, the background was obviously fake. (00:35:35)
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