Visible crew/equipment: When Phoebe's assistant walks into the office carrying a tub of mail for Phoebe, the shadows of the boom mic are visible on the wall over her and the TV. (00:00:25)
Womb Raider - S4-E21
Visible crew/equipment: While Piper, Phoebe, and Paige are in the magical cage, the Dark Priest, the demons, and the Seer are all killed/vanquished, then when Piper kicks open the cage the sisters walk out, and we can see the black mic transmitter attached to Phoebe's waist, at her back on the right. (00:38:40)
Visible crew/equipment: During Cole's meeting with Dane and several other demons, just as Dane begins to talk about Gregory Conroy being a probation officer to juvenile offenders, the yellow T-mark is visible on the floor near the actor's feet. (00:06:00)
We're Off to See the Wizard - S4-E19
Visible crew/equipment: After Phoebe tells Cole she's pregnant she leaves in the elevator, and when it cuts to Piper, Paige, and Leo we can see the crew's grey mat on the floor under Piper and Paige, then in two following shots (from the same angle) the red T-mark is visible on that mat, at Paige's feet. (00:07:45)
Continuity mistake: While at P3, Leo tells Phoebe that she can't turn into a Harpy just by getting scratched, and when Piper cautions Leo to use discretion Piper raises her left arm to Leo, but in the next shot it's Piper's right arm that's raised. (00:16:05)
Visible crew/equipment: When the Dark Priest finds the box containing the Lazarus Demon, just as the Seer says that she can always count on him to help, the shadow of the boom mic dips into view at the top of the screen. (00:09:05)
Visible crew/equipment: When Phoebe tells Piper that she's on a deadline and hangs up the phone, in the next shot as Piper says that she's going to Phoebe at the newspaper, the shadow of the boom pole/mic is visible on the wall behind Piper, at the top of the screen. (00:26:45)
Continuity mistake: After Phoebe finds the Scavenger Demon page in the Book of Shadows the camera dollies out, and when Piper asks what happened with Cole, the crew's taped black mat is visible on the bare floor with no fringed area rug, then when Cole shimmers into the attic, in the next shot of Piper and Phoebe we see the fringed area rug at the bottom of the screen. (00:06:35)
Visible crew/equipment: When Sykes is about to kill Susan at the ADA'S office, Leo hits Sykes and orbs out with Susan, and in the next shot as Sykes sits up we can see the green tape mark on the floor, under his leg. (00:22:30)
Continuity mistake: When Phoebe adds the photos to the Book of Shadows, Phoebe writes "Cole's human form" with large letters, then in the shot facing Phoebe as she says, "Your date, the guy you've been smooching," we see it's underlined with smaller writing, but in following shots it's back to the original larger lettering without the underline.
All Hell Breaks Loose - S3-E22
Visible crew/equipment: During the scene in the kitchen, after Piper reads through all the faxed interview requests, in the wideshots facing Darryl the reflection of the moving boom mic is visible on the glass at the top, center of the screen. (00:21:15)
Visible crew/equipment: After Piper summons Grams and then Patty appears as well, when it cuts to Darryl and Cortez entering the manor, the blue and white T-marks taped to the crew's black mat are visible, at the bottom of the screen. (00:34:00)
The Demon Who Came In from the Cold - S3-E19
Visible crew/equipment: After Prue removes the piece of Belthazor flesh from the freezer it cuts to Cole finding Phoebe's note in his pocket with a closeup of Cole, and at the start of the next shot the reflection of the boom operator holding up the boom pole and mic can be seen on the glass window, behind Pirelli/Vornac. (00:28:05)
Continuity mistake: After Prue and Phoebe send contagious Piper back up to bed, it cuts to three consecutive shots of buses arriving at camp. In the first shot, the bus license plate is white with the number "2IAB345" and we see markings on the bus as the counselor exits. The second closer shot shows two kids exiting the bus. Then in the third wideshot, with those same two kids beside the door, the license plate is now blue with the number "876QPD" and we see the same markings on the bus as in the first shot, so we know it's the same bus. (00:22:05)
Visible crew/equipment: After Lukas kills the other demon so he can move forward with his plan to go after the witches, when it cuts to the manor's foyer as Prue and Phoebe walk in, the green T-mark is visible on the rug at the bottom of the screen. (00:08:15)
Visible crew/equipment: After the Grimlocks enter the manor Leo calls for Piper then rushes out of the kitchen, and when it cuts to the shot of the dining room the edge of the crew's black mat is visible at the bottom of the screen. The black mat is again when the sisters are pulled into the time portal, though the only thing we should be seeing is the fringed area rug. (00:04:00)
Continuity mistake: While Dantalian tries to steal the Book of Shadows from the attic Piper and Phoebe surprise her, then when Phoebe kicks Dantalian and she falls we see the stuntwoman's footwear has chunky sole ridges, but when the sisters approach Dantalian her shoes have heels. (00:32:10)
Blinded by the Whitelighter - S3-E11
Visible crew/equipment: After the opening credits, when it cuts to Piper and Leo in the dining room the green tape mark is visible under the table, and the red T-mark and long blue tape mark are also visible on the crew's black floor mat behind Leo. (00:04:45)
We All Scream for Ice Cream - S3-E10
Visible crew/equipment: When Phoebe gets an IM on the laptop Prue walks over to her, and just as Prue sits down at the table we can see the blue tape mark on the floor, where she had been standing. (00:05:50)
Visible crew/equipment: After following the signs that lead to the demolition site, when Prue unlocks the chain on the door and enters the loft, the T-mark is visible on the floor at her feet. (00:03:45)
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.