Used Karma - S6-E13
Continuity mistake: In the scene when Piper is fighting off demons by herself at the start of the episode and Phoebe and Paige come to help her, she narrowly misses a fireball and jumps down to the ground to dodge it. In the shot before, her forehead is un-injured, but as she dives to the ground you see that she is already bleeding from her forehead, *before* she actually hurts it. (00:02:30)
Used Karma - S6-E13
Continuity mistake: In the scene when Paige and Piper are doing first aid on each other, Piper makes quite a production out of the business of peeling the backs off band aids to apply to Paige's arm - but when she slaps it on Paige's arm, it's a non-adhesive white 4 x 4, which Paige has to hold on her arm with her other hand.
Used Karma - S6-E13
Continuity mistake: At the beginning, Piper is wounded on her forehead by the demons. When they orb out and orb into the kitchen Piper doesn't have a wound on her head anymore. (00:03:25 - 00:05:40)
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