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.
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.