Visible crew/equipment: While Billie is in the dorm room with her friend, Taylor, talking about the disappearances, when Taylor gets up and stands beside Billie, the pink tape mark (it's a toe box mark) is visible on Billie's seat. (00:07:20)
Repo Manor - S8-E13
Other mistake: When Piper blows up the dollhouse, killing Pilar, Phoenix and Patra, you can hear the sound effect of telekinesis, despite Piper's power being molecular combustion.
Other mistake: When The Angel of Death comes to Leo in the car Leo asks him "can I at least see my son again?" which is an odd way to ask that question given that he has two sons.
Hulkus Pocus - S8-E9
Other mistake: When Piper is explaining to Phoebe that they don't have enough time to save Billie, she lists all the things they need to do ending with "fix us and then fix Billie..." This doesn't make any sense, since when she's saying it there's nothing wrong with them to fix. It's only after she says this that they come up with the plan to infect themselves. (00:29:54)
Plot hole: In the final episode Piper cast the spell with her mother Patty and grandmother Penny to get the Hollow out of (past) Piper, Phoebe, Paige, Billy and Christy. It worked. But in episode 4x13 Charmed and Dangerous, the only way to contain the Hollow is by good and evil working together... which wasn't the case in the final episode.
Continuity mistake: Season 8, Episode 14, "12 Angry Zen": When Phoebe finds Paige at the manor after the demon/Zodiac fight, Paige rubs her neck with her left hand. In the next shot, it's suddenly her right hand on her neck. (00:28:55)
Answer: "Hush, Hush, Hush" Paula Cole.