Power Outage - S3-E7
Revealing mistake: Alyssa's tattoo is visible when Phoebe leaves Cole's apartment after she fought with her sisters.
Sleuthing With the Enemy - S3-E8
Revealing mistake: When Cole / Belthazar grabs Phoebe to use her as a hostage, you can see, very obviously, that he doesn't pull her. Phoebe actually steps in front of him willingly, despite the look of fear on her face.
Sleuthing With the Enemy - S3-E8
Revealing mistake: When Phoebe throws the potion at Cole's coat, the vial doesn't shatter, even though it sounds like it. Watch closely, and you'll see the purple, unbroken vial bounce off the coat and away out of shot without any of the potion touching the coat.
Coyote Piper - S3-E9
Revealing mistake: Prue knocked Piper unconscious in the cat fight (after the Coyote Dance), and asked Justin to carry her to the car. But when he is apparently lifting Piper up from the floor, you see Piper standing up by herself.
Coyote Piper - S3-E9
Revealing mistake: After Phoebe has her premonition in the mausoleum, she wipes away the dust from the crypt to reveal Cole's father's name. Just BEFORE they show her wiping the dust away, you can see the letters in the shot of her leaning against the crypt.
Revealing mistake: When Prue talks about her relationship with Justin near the beginning of the episode, she says that she finds him to be very predictable, lacking in mystery or "savoir-faire." That doesn't make sense, though, as "savoir-faire" means "know-how." She probably meant "je ne sais quoi," that is, "I know not what."
Pre-Witched - S3-E17
Revealing mistake: At the beginning of the episode Piper and Leo are in the shower together - at the bottom of the screen you can see Piper has on a flesh colored towel.
Revealing mistake: The sound of Phoebe writing with a permanent marker was added in, as you can see that the marker never touches the paper. Then after a few seconds Phoebe stops writing to listen to Prue and her marker isn't moving but the sound of writing can be heard. (00:05:30)
Revealing mistake: When Phoebe and Paige use Cole as bait to lure Piper to the manor, Phoebe levitates and kicks two furies at once. When they show the front shot of her kicking these two furies, we can see Alyssa Milano's stunt double. (00:34:25)
Revealing mistake: In the beginning of the episode, Yenlo and Anling are fighting with swords. After Yenlo says, "I'm going to kill him, Anling. And then I'm going to kill you," Anling stabs him with her sword. Yenlos' wound is very bloody, but there is no blood on Anling's sword at all. (00:04:25)
Revealing mistake: When the prince is supposed to be frozen on Paige's living room couch, when they do a close-up you can see the actor blink.
Brain Drain - S4-E7
Revealing mistake: When the Chameleon Demon changes from a lamp to a demon he throws Piper and Phoebe with Telekinesis but Piper looks to the right before he motions the toss. (00:03:05)
Revealing mistake: When Paige cuts the flesh away from Sykes, you can visibly see a white bandage-like thing under his skin as it comes off. (00:23:20)
Charmed and Dangerous - S4-E13
Revealing mistake: In the scene where The Source uses Piper's powers together on the clay object (it blows up and then freezes), after he unfreezes it, the pieces shatter and one lands on the stand thing. As the camera moves in towards The Source, before the camera passes the clay piece it visibly vanishes (as it is CGI) and then the Source holds up a piece which is much larger and a different shape. (00:19:45)
Charmed and Dangerous - S4-E13
Revealing mistake: During the episode, Paige is wearing a sleeveless top, but when she thrown across the attic by 'The Source' the stunt double is wearing sleeves. (00:37:45)
Charmed and Dangerous - S4-E13
Revealing mistake: At the end of the episode after Phoebe and the Seer put the hollow away in its place, she leaves, but when it next cuts to a closeup of Cole, in the corner of the screen you can see her hair. (00:41:50)
Revealing mistake: In the teaser, when Leo levitates to hang a picture, he has a major wedgie and the ankles of his pants hike up to mid-shin due to the harness that is used for the levitation effect.
Revealing mistake: When Piper blows up the painting you can see the hole used to put the charge into to blow it up. (00:57:40)
Revealing mistake: When Phoebe goes to sign her papers at the end, she spells her name as Pheobe. (01:22:45)
Siren Song - S5-E4
Revealing mistake: When the Siren is being healed, you can see that she's just holding the pole next to her, when it's supposed to be going straight through her. (00:19:50)
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.