Little Box of Horrors - S7-E18
Character mistake: Katya (as Nina) says that Pandora's Box is "a gift the gods gave to Prometheus." But the Box was given to Epimetheus, not Prometheus. Epimetheus ignored the warnings of Prometheus, his brother, and accepted Pandora herself, who bore a box, as a gift from the gods.
Character mistake: In the attic, Melinda Warren is awed at how thick the Book of Shadows has gotten. But according to information from previous episodes, the Book was started after Melinda had died, so she wouldn't know how thick or not it was.
Witchstock - S6-E11
Character mistake: Phoebe, who has moved to Hong Kong, tells Paige that she, Paige, was born in the Year of the Ox. But Paige was born in 1977, the Year of the Snake. 1973 was the Year of the Ox.
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)
Chick Flick - S2-E18
Character mistake: After the horror movie characters were killed in the attic, the sisters hear the door bell ringing. When they get to the bottom of the stairs and start walking towards the front door, you can see the actor playing Billy smiling even though they're all supposed to be on edge.
Character mistake: When Miles is shot, the cops apparently don't think it's necessary to tend to him to see if he's okay.
Charmed and Dangerous - S4-E13
Character mistake: When Phoebe is describing to Cole the demon she saw in her premonition, she says on the good side of his face he had tribal markings, and Cole says, "Kinda like Belthazar," and Phoebe agrees. Belthazar has no markings like the ones on the demon's face.
Character mistake: Paige goes undercover into the demonic market wearing a bright blue dress and blond wig, and none of the demons seem to notice she sticks out like a sore thumb.
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.