Plot hole: It's shown with the spell that the Halliwells leave for Chris in the attic that if something happens in the past, the future will immediately reflect that change. Chris had changed so much in the past that there should have been at least one thing different that mildly surprised him, even if he was careful to not give too much information to the sisters about the future.
Continuity mistake: In this episode, Chris thinks back to when he went back to the past. When he and Bianca are on the tour of the manor, there are some costumes that have been worn by the charmed ones. But in the episode "Oh My Goddess (1)" Chris tells Phoebe that Paige died on that day, so how could she have worn something after she died? And if she did die that day, Chris would have never met her, since he was not yet born, and could not have learned to orb from her, as he says in "Spin City." However there are other inconsistencies with that story that Chris mentions later, seeming to suggest he lied about this to get the sisters onside. It may not be a continuity error but as it was never fully addressed in the show just what Chris was lying about (and he obviously told some) we cannot be sure.
Revealing mistake: In Chris's flashbacks, it's supposed to be the day he left to come to the future. But his hair is longer than it was in the season 5 finale when he came to the future.
Continuity mistake: When Chris and Bianca go to the manor to get Chris to go to the past, which would end up him appearing in "oh my goddess"; he's only wearing a black shirt and pants. When he arrives to the past in "oh my goddess" he's wearing a jacket, red shirt, and sunglasses. He wasn't wearing that in Chris-Crossed. And he didn't have time to change to that outfit.
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