Continuity mistake: When Gideon warns Sara not to use magic to put the books in the library back on the shelves, her arms are at her sides. A second later, the shot changes to over her shoulder as Leo orbs in and she has a nearly full armload of books.
Other mistake: When the Darklighter shoots Leo in the attic on the Ghost Plane, the arrow goes all the way through him. He wasn't shown to be using anything other than a normal Darklighter Crossbow, and there wasn't any evidence that, since Leo and Piper were still alive, that the physics of the Ghost Plane are any different from the Normal Plane. Yet, in every other instance of someone getting shot with an arrow from a crossbow, it never, ever penetrates the person's body, and always get stuck.
Revealing mistake: Just after Piper and Leo are taken away, Paige orbs down to the ground and runs up to Pheobe, places her hand on her, and shows a blood-covered hand showing that Pheobe's been injured. But before it's shown, while Paige is running up to Pheobe, you can see Paige deliberately keeping her hand closed and facing away from the camera, because it's already there when she orbs down.