Continuity mistake: In the teaser, Phoebe has a vision of a murder. When she first sees the shadow of the attack, the assailant is holding his victim down with his right hand while stabbing with his left. Then when the vision focuses on the Medusa head tattoo on his forearm, it is the right arm that is stabbing.
Plot hole: When Leo tells Piper that he believes he found the demon Sargon in the Book of Shadows (which may be the demon Count Roget made a faustian deal with), Piper looks at it and says that they vanquished this demon five years ago. Which would've meant they vanquished the demon with Prue. When in fact the only demon that happened with was Zahn in season 6 episode 6, "Soul Survivor," and they did that with Paige.
Continuity mistake: At the beginning of the episode they show Piper running up the stairs to her house in the rain. The umbrella she has is black, with a colored ring around the edge. When she enters the house the whole umbrella has changed to colors surrounding the whole umbrella now.
Other mistake: After Piper freezes the first crime scene that Cole and Phoebe (aka Frankie and Lulu) have hit, Piper unfreezes Darryl by touching him but at the beginning of the the episode when Piper and Cole are touching the car that Piper froze, to stop it from exploding after a car crash, while trying to help the woman inside, the car never unfreezes. This is somewhat inconsistent through the entire show a lot of times they can unfreeze things by touching them and then a lot of times they can only unfreeze things by Piper using her pointer finger to either unfreeze a head or a foot or some other body part.
Continuity mistake: In the second to last scene where Piper tests her powers on the lamp and Phoebe is cleaning, you can see Piper's bra strap when she blows up the lamp but when she turns to face Phoebe its covered. Through out the scene it's covered and exposed without her ever adjusting her shirt.