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.
Factual error: Considering how the sisters and Cole every now and then - especially during seasons 3-5 - find themselves in some churchyard or cemetery, it should be noted that there are no such places in San Francisco! Except for the original one at an old nunnery, from before the city was founded, and the Presidio Military Churchyard, all graves were moved in the first half of the 20th Century to the cemeteries at Colma, a few miles south of SF. This was done due to the sanitary risks in case of earthquakes, and the lack of space for churchyards downtown.
Continuity mistake: When Phoebe is walking the Dwarf to the door after finding out Piper pulled the sword from the stone, he steps outside and goes into fast-forward, then stops and goes back to normal speed as he sniffs a flower. During this, in the background, you can see Phoebe, who freezes the whole time.
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.