Continuity mistake: When Prue runs into Susan Trudeau at the restaurant, she asks if she's Andy's sister, only to find she's his ex-wife. But Prue knew Andy all through high school - she would have known he didn't have a sister.
Other mistake: When Piper and Mark are in the police parking lot she holds the door open for him to get into her SUV. When he gets in you can see the SUV go down from his weight in the vehicle. Ghosts are supposed to be weightless.