Continuity mistake: Episode 8x17, "Generation Hex": When Phoebe hands Paige her wedding gift, the way Paige holds it changes between shots. (00:01:20)
Continuity mistake: When Candor interrupts the Noxon demons torturing a student at Magic School, Rondok turns around to look at him. In the following shot, he's facing the other way and turns around again. (00:06:10)
Continuity mistake: When Phoebe and Cupid visit the memory of Phoebe revealing she is a witch to Dex, past-Phoebe has her hands down by her sides, but then her left hand suddenly jumps to her purse strap. Not only that, but in the original episode ("Rewitched"), Phoebe's hand remained on her purse strap the entire time. (00:11:05)
Gone With The Witches - S8-E20
Continuity mistake: When the leprechauns are called back to the manor from Magic School, O'Brien cradles his injured arm in his left hand. In the next shot, his arm is down by his side and he moves it to his arm again. (00:25:10)
Plot hole: When Piper goes back in time to save her sisters from dying and get rid of the Hollow, right after Leo describes it as "time catching up with itself" and the new reality Piper takes over her body. Yet in Season 5 Episode 7 "A Witch in Time", Piper goes back to keep Phoebe from saving her boyfriend but once Piper changes the past, the original Piper fades away and the new reality Piper stays.
Plot hole: In the final episode Piper cast the spell with her mother Patty and grandmother Penny to get the Hollow out of (past) Piper, Phoebe, Paige, Billy and Christy. It worked. But in episode 4x13 Charmed and Dangerous, the only way to contain the Hollow is by good and evil working together... which wasn't the case in the final episode.
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.