Other mistake: In this episode, the Source talks about the "Ultimate Power" being the Hollow. But in Season 8, we learn that the "Ultimate Power" is Billy and Christy. (And they end up taking in the Hollow, so now are they the "Ultime Power" times 2?) Which is it? It can't be both.
Character mistake: When Phoebe is describing to Cole the demon she saw in her premonition, she says on the good side of his face he had tribal markings, and Cole says, "Kinda like Belthazar," and Phoebe agrees. Belthazar has no markings like the ones on the demon's face.
Revealing mistake: In the scene where The Source uses Piper's powers together on the clay object (it blows up and then freezes), after he unfreezes it, the pieces shatter and one lands on the stand thing. As the camera moves in towards The Source, before the camera passes the clay piece it visibly vanishes (as it is CGI) and then the Source holds up a piece which is much larger and a different shape.