Other mistake: Prue tells Andy about her powers and he asks her if when she has kids they would inherit them - she says "If they are girls, yes" but later on in the series Piper has two boys who are both very powerful. (00:30:30)
Continuity mistake: You'll notice in some episodes (like "Coyote Piper") that when the sisters look out of the attic window, they can see Prescott Street below, indicating that the attic window is on the front of the house. But if you look at the size and shape of this window, you'll see that there is no window like this on the outside front of the house. In addition, in "Sin Francisco," Phoebe looks out the attic window and sees the front porch from a side view. There's no structure on that side of the house to begin with.
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.
Revealing mistake: Throughout the series, Finola Hughes appeared as Patty, the deceased mother of the Charmed Ones. Although Patty died in 1978, she sometimes appeared in flashbacks and time-travel episodes that took place in the mid to late 1970s, and other times she appeared as a ghost. However, over the eight years of the series, Finola Hughes aged noticeably. Patty was born in 1950, so she should always have been either a young woman in her mid 20s, or an unaging ghost having died at 28.
Factual error: Throughout season 8 the members of the Triad wore a pentagrams on their gowns that would alternate from the point being up and point being down. They should only be wearing a pentagram with the point facing down. Pentagrams with the point facing up is the symbol for practitioners of white (good) magic while those who practice black (evil) magic is symbolized by the point facing down.
Factual error: Numerous times in the establishing shots of San Francisco, the Embarcadero Freeway is visible. While the bridge did run along the San Francisco Bay, it was demolished in 1991 - seven years before the series even started.
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.
Something Wicca This Way Comes - S1-E1
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. (00:02:35)
Something Wicca This Way Comes - S1-E1
Visible crew/equipment: When Piper and Phoebe are standing beside the car Piper kisses "Nancy Drew" goodbye, and Phoebe continues to tell her sister, "We're known as the Charmed Ones," then in the next overhead shot the reflection of the boom pole/mic is visible on the car's windshield, just as Piper drives off. (00:20:10)
Something Wicca This Way Comes - S1-E1
Continuity mistake: In the scene when Piper and Jeremy are in the taxi, Piper gives him a fortune cookie. You can see her closing the box. In the next shot of Jeremy, you can see the box is still open. (00:31:15)
Something Wicca This Way Comes - S1-E1
Visible crew/equipment: When Jeremy wakes up in the freight elevator he rushes off to find Piper, and when it cuts to Piper and Prue in the living room, the reflection of the boom mic is visible on the glass of the framed photo, behind the couch. (00:36:20)
Something Wicca This Way Comes - S1-E1
Continuity mistake: At the end of the episode the newspaper Prue is holding keeps changing from facing the camera to facing her. And as she turns to the house she drops her hand (and the newspaper) to her side but in the next shot she is holding it to her chest again. (00:40:25)
Something Wicca This Way Comes - S1-E1
Continuity mistake: At the beginning of the episode we see a man standing in the rain in front of the sisters house. After he starts walking away, they show a completely bare tree in front of the house. The next morning when Piper is driving away, they show the same tree with leaves and flowers on it. Trees do not grow that fast.
Something Wicca This Way Comes - S1-E1
Continuity mistake: When Piper and Jeremy are in the elevator, he pulls a knife on her she freezes him. When the camera shows him from her p.o.v he has this really "snarling" looking face with his mouth open. But when the camera changes the shot from looking at them from outside the elevator, his face is completely different and his mouth is almost closed.
I've Got You Under My Skin - S1-E2
Continuity mistake: When Phoebe flirts with Alec at Quake, she rests her head in her hand. In the following shots between Alec and Phoebe, her hand moves back and forth from her face to the tabletop. (00:02:00)
I've Got You Under My Skin - S1-E2
Visible crew/equipment: When Prue walks into the manor and sees Brittany in the foyer, we can see the boom pole moving at the top of the doorway leading to the dining room. (00:35:20)
I've Got You Under My Skin - S1-E2
Continuity mistake: In the previous episode, Piper tried out for a job as a chef at a restaurant called "L'Opera Ristorante" and was hired by Chef Moore. Then in this episode, she works at a bar and restaurant called "Quake," but she talks about Chef Moore quitting and leaving her in charge of the establishment, so it's not meant to be a different place.
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.
Thank You for Not Morphing - S1-E3
Continuity mistake: When Prue tells Victor to leave the auction house, Victor has his arms behind his back, but in the next shot, his arms are in front of him. (00:09:40)
Thank You for Not Morphing - S1-E3
Visible crew/equipment: In the scene where Prue comes home and finds Andy inside the manor. There is a shot where if you look behind Andy's shoulder you can see half of the body of the director in the background in the observatory room sitting in a director's chair. (00:16:00)
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.