Revealing mistake: When Prue is practising using her powers while astral projecting, when Phoebe walks away to go see her in the attack you can see the makeup they used in an attempt to hide her tattoo on the right side of her back. It looks as though she's been bruised there. (00:00:43)
Siren Song - S5-E4
Character mistake: When discussing how best to help Melissa, Piper tells Paige that her birth father was half whitelighter, when he was fully a whitelighter. (00:25:15)
Witch Wars - S6-E21
Continuity mistake: When Paige goes to The Bay Mirror to bring Phoebe potions we see she has 3 different colored potions: green, blue, and orange. She hands the light blue "explosive" to Phoebe, then hands the green one saying it's "acid" but when she hands Phoebe the last potion the camera angle changes and we see that she's again handing her the green one. At this time we see that Phoebe is holding the blue and the orange ones. (00:10:28)
Plot hole: When the Avatars agree to rewind time they say that they can either put the world right or save Brody but not both. This doesn't make sense since they're rewinding to right when they put everyone to sleep, which is before Brody was killed.
Other mistake: When Phoebe gets the premonition to see all the loss, she sees the scene where Piper dies in All Hell Breaks Loose. Since time was rewound this event never happened in this timeline, and we're never given any reason to believe Phoebe would know this happened after the time reversal.
Other mistake: After they talk to the Angel of Destiny for the first time, they send Leo to ask the Elders if he is legit. After Leo leaves they find out about Agent Jackman, and the following events unfold. After Jackman busts them Piper calls for Leo and tells him "It's OK, you can orb" and he appears asking her "What about Agent Jackman?" Except that Leo was gone before they found out about him and the girls hadn't told Leo yet that he even existed. (00:09:09 - 00:21:58)
Other mistake: When Phoebe is trying to summon Angela the spell doesn't work and Leo says that she needs the power of three. We've seen this spell done with just one sister in the past and it's never explained why they'd need to do it together this time.
Continuity mistake: The furniture used throughout the show regularly gets blown up or destroyed but then mysteriously re appears later on or in the next episode.
A Wrong Day's Journey Into Right - S6-E20
Revealing mistake: Right after Paige is orbing out, the camera is on Phoebe and on her right, you still see Paige's shoulder. And then it cuts to Paige orbing back in.
Factual error: During the ritual, we can see that the snake that is magically transformed into a man is a milk snake. But it's not poisonous, it's a colubrid. The creators apparently wanted to use its resemblance to a poisonous coral snake, which is an elapid. (00:02:35)
Continuity mistake: When Paige is at the bar with her newest charge we see the same extra walk by 3 times, going the same direction each time.
I've Got You Under My Skin - S1-E2
Revealing mistake: At the end of the episode when Phoebe and the other two are at the bar you can notice that Prue straw in her drink goes in and out of her drink a great deal.
Continuity mistake: When Prue lets Matthew out of the locket, the front view Prue has her right hand on her stomach, when the shot changes to the back view her hands are by her side, it does this multiple times.
Something Wicca This Way Goes - S7-E22
Continuity mistake: When Zankou is looking at the spell in the Book to steal TCO powers we see that it is the same spell Aunt Gail used in "How to Make a Quilt out of Americans", except that in that episode she ripped the page out of the book. It shouldn't be there for Zankou to use. (00:09:56)
Visible crew/equipment: When Piper is coming down the stairs with the staff, if you look in the top left corner of the stairs, you can see a piece of camera equipment moving.
The Bare Witch Project - S7-E2
Other mistake: At the end of the episode when Phoebe is on the horse making her lady Godiva speech her jeans show at her waist. When they pan out she is "naked." When she starts to ride away her skin tone underwear shows. They cut away to Paige and Piper and when they come back to Phoebe riding away the underwear is covered by the long blond hair. (00:41:05)
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)
It's A Bad, Bad, Bad, Bad World (1) - S6-E22
Other mistake: Phoebe says there's nowhere to park except blocking the neighbor's driveway. We can see in the opening shot that there is no-one parked in front of the manor, and we see spots across the street. There is also enough room to park where she is without blocking the driveway. At the end of the episode, despite not moving it and also saying that she hasn't, it's not blocking the driveway any more. The patrol guy still shoots her. It's not illegal to park there unless it's blocking the driveway.
Answer: That was a mistake in the series.
Also, the episode is called 'Babies First Demon' and this alone tells us that it's Wyatt's first time in the field (so to speak) and as the sisters say, they've done a good job of clearing up their own magical messes so far. Nobody really knows how advanced Wyatt's powers are, or will become. That's why the cleaners appear, to nip things in the bud. The Cleaners didn't want to chance that the sisters could clean Wyatt's magic up, just incase they couldn't. Therefore intervening themselves, just in case.