Continuity mistake: After class, when Phoebe is talking to her study group she pulls out the flyer for P3, and in the first shot the flyer is in Phoebe's right hand, but in the next shot she's holding it out in her left hand. (00:01:40)
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: After the manor is broken into, in the following shots while everyone is in the foyer the left front door is wide open in most shots, but in some other shots both front doors are closed during this scene. (00:21:50)
Continuity mistake: When the ani-men break into the manor the entire large center glass of the left door ends up broken, but later while Phoebe is looking for the hydrogen peroxide, Prue notices the front doors and the left door's center glass is intact with only the bottom corner broken. (00:22:00 - 00:27:05)
Visible crew/equipment: After Phoebe has turned everyone in P3 into animals, when Phoebe stands at the bar the white T-mark is visible on the floor at her feet, twice. (00:35:00)
Continuity mistake: When Prue is sitting at home bored, she spots the vase with the flowers. In one shot the vase is on the left, and in the next shot it's on the right, then Prue moves it to the left.
Revealing mistake: When Phoebe opens the cupboard and the human pig falls on top of her, you can see that it's a stunt double in the far shot.
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.