Love Hurts - S1-E21
Revealing mistake: After Piper saves Leo, they meet with Phoebe and Daisy downstairs near the living room. Before the darklighter appears, smoke from a fog machine can be seen at the left of the shot to magically transition to the computerized effects. (00:33:45)
Love Hurts - S1-E21
Continuity mistake: Leo dies and then Piper heals him. In all future episodes it's made very clear that Leo's powers can't heal the dead.
Deja Vu All Over Again - S1-E22
Continuity mistake: At the end of the episode, Phoebe's hairstyle changes from tied back when talking to Piper in the kitchen, to down and re-done in the time from the kitchen to the front porch.
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