Plot hole: Why doesn't Chris ever use his powers against demons? We know he can use them in this time, since he blows up Leo in "Oh My Goddess" and kills a Valkyrie in "Valhalley of the Dolls", and after "Chris-Crossed", when it was discovered he was half-witch he had no reason to hide them from the sisters, yet he never once used them against a demon, even when one of the Charmed ones is in mortal danger.
Continuity mistake: When Piper, Paige and Phoebe are having a food fight, Piper freezes the cake, then she throws it at Paige who suddenly has a mysterious stain on her top. Then Piper has orange food on her hands even though she hasn't touched anything orange, and the stain on Paige's top has gone.
Plot hole: This episode takes place in 1967, and Penny (Grams) is married to a man named Allen Halliwell who dies in the episode. Allen is supposed to be Penny's "first husband," Patty's father and the Charmed Ones' grandfather. But it was previously established that Patty's father was named Jack, and he died in 1964. Also they hint that Patty is still little but we know she was born in 1950 so she would be 17.
Plot hole: In the opening scene, Chris directs Piper and Phoebe on how to vanquish the amorphous demon. He says that it will attack "fast and hard" as soon as it senses Phoebe's magic when she levitates. But Chris had orbed out of the warehouse and back in, and the demon didn't react to the magic. Later in the episode, it is specifically attracted to a use of orbing, several times.
Plot hole: Rick is menacing Phoebe with a gun near the end of the episode. Why doesn't Paige orb the gun away from him? In fact, why did Phoebe make Piper stay upstairs? Piper could have frozen Rick, and Paige could have orbed him back to the prison. There were several obvious ways to resolve this situation that would not have caused Rick's death.
Continuity mistake: Paige's spell causes Darryl to acquire invulnerability and uncontrollable super strength. When he opens the door of the squad car, he accidentally rips it completely off. When he tries to put the suspect into the back seat, the man flies through the car and barrels out the other side, taking the other door with him. But immediately before this, Darryl high-fives several of his fellow officers, and they are unharmed. They should have been thrown off their feet and suffered multiple fractures in their hands.
Continuity mistake: The Stillman sisters strip the Charmed Ones' powers by using the same spell as Aunt Gail did in Season 2 (How to Make a Quilt Out of Americans). But in that episode the Charmed Ones had to first drink a potion and Aunt Gail needed to wait a certain amount of time (at least 30 minutes) before the spell worked. In the Power of Three Blondes, the Charmed Ones never drank a potion and lost their powers before the Stillman sisters even finished the spell.
Continuity mistake: Just after Olivia has killed Mr. Montana and everyone goes running up to see what has happened, Phoebe waves to Paige through the window of the Calloway's house which is shown to be right next door to the Montana's house. Earlier in the episode it had been established that they lived across the street.
Continuity mistake: During the scene when Paige goes to the citrus plant during the labor rally, her pink blouse is quite short, baring her entire midriff. When the security guard throws Paige out of the plant, Rose McGowan's stunt double is wearing a shirt in the same pink color, but it reaches all the way down to her waistline.
Plot hole: When Gith takes the potion from Chris, he ends up stomping on it and shattering the potion bottle - making contact with the potion. We've seen that potions don't have to be thrown by someone who's even a witch for them to work (like when Cole's powers were stripped etc) so why on earth would he do this? And how did it not vanquish him? There are other ways he could have gotten rid of the potion that didn't involve a way that really should have vanquished him.
Other mistake: In previous episodes it is stated that good witches don't freeze. But after Piper has been decapitated and her head is placed in the manor, one of the students freezes all the kids by tapping into Piper's freezing power. The kids are all good witches so they should not have frozen.