Continuity mistake: After the potion is spilt, the green stain it leaves on Phoebe's skirt keeps changing both size and colour. (00:31:30 - 00:34:35)
Continuity mistake: After Phoebe tells Piper about the balloon plan, when Piper starts talking about Leo she fills the blender on the counter, and in the two shots facing Piper the white lid is on top of the blender, but in the shot facing Phoebe the lid is gone before she turns it on. (00:19:10)
Continuity mistake: When Leo comes to the manor to tell Piper about Masselin, during the conversation Prue is not wearing her choker in the shots facing her (with her arms crossed) and Phoebe, but in the next shot when Leo tells Piper that he knows she's upset we see Prue's choker suddenly around her neck. (00:11:50)
Visible crew/equipment: After Prue gets off the phone with Barker and tells Phoebe about it, when Piper runs into the kitchen with the news about Dishwalla, the reflection of the moving boom pole can be seen on the glass cabinet. (00:10:00)
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.