Continuity mistake: Piper does not light the candles before she summons Grams, but in the episode 'PreWitched' (when Grams summons Patty) we see that the candles must be kept alight to maintain the connection with the dead. (00:18:00 - 00:33:30)
Continuity mistake: After Phoebe has her premonition about Paige at Prue's funeral, demonic bounty hunters show up to "eliminate" Belthazor. We see the funeral hall empty where seconds before Phoebe's premonition, there was a line of people waiting to give their condolences to the girls, Victor, Leo and Cole. (00:12:20 - 00:13:10)
Continuity mistake: During Prue's funeral when the demonic bounty hunters attack, one gets thrown into some chairs, which knocks some down. When we see the chairs again in another shot the ones left standing and the ones knocked down have changed. (00:13:25)
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