Sleuthing With the Enemy - S3-E8
Other mistake: Cole is supposed to be one of the most powerful demons alive, but a small cut from Piper weakens him to the point where he can't even shimmer or keep up his "Cole" appearance.
Continuity mistake: When the banshee came to the attic, she smashed all of the windows, except for some in the middle of the big window, but in the next shot that glass was gone.
Audio problem: When Phoebe hits Cole over the head in the courtroom, the sound of something unfreezing is heard, but Cole wasn't frozen and Piper doesn't unfreeze the room until a few moments later.
Continuity mistake: In the scene where Cole has caught Phoebe's karate kick at school, Phoebe's hands are both clutching her school books. In the following shot, her left arm is in a completely different position, and there's no reason for her to have moved it. (00:18:25)
Sight Unseen - S3-E5
Factual error: In the scene where the invisible demon is entering Cole's office, the courthouse shown before the elevator is a New York City courthouse, not San Fransisco. It's confirmed by the presence of a NYC Taxi.
Continuity mistake: In the scene where Eva is teaching the sisters about ways to draw power on All Hallows Eve, she cuts an apple in half and puts a leaf over the seeds in the center. The leaf goes from small and only covering the center to bigger and covering more of the apple.
Wrestling with Demons - S3-E12
Continuity mistake: When Phoebe finds the lost and found spell for Piper, the paper the spell is written on moves relative to the top of Phoebe's keepsake box. (00:10:30)
Magic Hour - S3-E2
Visible crew/equipment: When Prue and Phoebe see Piper arrive home, a boom mic dips into the shot. (00:01:30)
Continuity mistake: In the season 2 finale the council has five members and yellow robes, in this episode they're now called the triad and there are 3 members in red robes (changed to black robes with inverted pentagrams in later seasons).
Audio problem: When Evil Prue flings Piper and Phoebe back, you see Piper hit a pillar, but with Phoebe's voice of pain. When we see Phoebe fall behind a structure, we hear Piper's voice of pain.
Sleuthing With the Enemy - S3-E8
Audio problem: When Phoebe fights with Krell in the mausoleum, the vial containing the Belthazor vanquishing potion is heard smashing, but after Krell is vanquished, Phoebe picks up an unsmashed vial and uses it to stage Cole's escape.
Continuity mistake: When Prue, Phoebe, and Leo enter the kitchen to find Piper covered in watermelon, the amount of fruit on Piper changes in the next shot. Her hands also change from being supine to prone. (00:19:00)
Continuity mistake: When Piper accidentally blows up the teapot, she has her hands up by her chest, in Prue's, but in the next shot her arms are down by her sides. (00:19:55)
Continuity mistake: When Piper accidentally destroys the clock at the passport agency, she freezes in fear with her hands near her face. In the next shot, her left hand is suddenly back by her ponytail. (00:13:40)
The Demon Who Came In from the Cold - S3-E19
Continuity mistake: When Phoebe and Cole are talking about him going undercover into the Brotherhood, Phoebe's grip on her pestle changes back and forth between shots. (00:10:15)
The Good, the Bad and the Cursed - S3-E14
Continuity mistake: After Cole kills the outlaw with an energy ball, he turns around to face Prue. In the following shot, he's facing the other way and turns around again. (00:39:45)
We All Scream for Ice Cream - S3-E10
Continuity mistake: When Victor is saying goodbye to Prue, Piper, and Phoebe, Prue has her hands in her jeans pockets, then crossed in front of her, then back in her pockets. (00:40:45)
Power Outage - S3-E7
Continuity mistake: When Cole drops Phoebe off at the manor after their date, Phoebe lowers her hand from Cole's face when they kiss, but then it's suddenly back up near his chin. (00:22:20)
Blinded by the Whitelighter - S3-E11
Continuity mistake: When Phoebe is looking in the Book of Shadows for information about Eames, she comes across the Belthazor page, and lays the page down flat. In the following shots, the page moves to her hand, then lying down flat, then to her hand again. (00:19:10)
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.