Revealing mistake: When Prue is practising using her powers while astral projecting, when Phoebe walks away to go see her in the attack you can see the makeup they used in an attempt to hide her tattoo on the right side of her back. It looks as though she's been bruised there. (00:00:43)
Siren Song - S5-E4
Character mistake: When discussing how best to help Melissa, Piper tells Paige that her birth father was half whitelighter, when he was fully a whitelighter. (00:25:15)
Revealing mistake: The character Ramus (the elder) changes actor when Leo jumps and orbs to get him away from the attack of the Aggressor. The actor's stand-in is visible instead. (00:18:52)
Continuity mistake: Leo says that Piper's past lover is his past life. If it was a past life, in 1924, that means Leo must have died almost straight away to be old enough to qualify as a doctor in WW2. And it wouldn't be his past life but his own past if it was the same life from before he became a Whitelighter. But then he'd also have been 15-20 years older before he died in WW2.
Witch Wars - S6-E21
Continuity mistake: When Paige goes to The Bay Mirror to bring Phoebe potions we see she has 3 different colored potions: green, blue, and orange. She hands the light blue "explosive" to Phoebe, then hands the green one saying it's "acid" but when she hands Phoebe the last potion the camera angle changes and we see that she's again handing her the green one. At this time we see that Phoebe is holding the blue and the orange ones. (00:10:28)
Wicca Envy - S1-E10
Audio problem: When talking about the tiara with a security guard Prue's mouth stops moving mid sentence but the audio continues.
Plot hole: When the Avatars agree to rewind time they say that they can either put the world right or save Brody but not both. This doesn't make sense since they're rewinding to right when they put everyone to sleep, which is before Brody was killed.
Other mistake: When Phoebe gets the premonition to see all the loss, she sees the scene where Piper dies in All Hell Breaks Loose. Since time was rewound this event never happened in this timeline, and we're never given any reason to believe Phoebe would know this happened after the time reversal.
Other mistake: After they talk to the Angel of Destiny for the first time, they send Leo to ask the Elders if he is legit. After Leo leaves they find out about Agent Jackman, and the following events unfold. After Jackman busts them Piper calls for Leo and tells him "It's OK, you can orb" and he appears asking her "What about Agent Jackman?" Except that Leo was gone before they found out about him and the girls hadn't told Leo yet that he even existed. (00:09:09 - 00:21:58)
Other mistake: When Phoebe is trying to summon Angela the spell doesn't work and Leo says that she needs the power of three. We've seen this spell done with just one sister in the past and it's never explained why they'd need to do it together this time.
Continuity mistake: The furniture used throughout the show regularly gets blown up or destroyed but then mysteriously re appears later on or in the next episode.
A Wrong Day's Journey Into Right - S6-E20
Revealing mistake: Right after Paige is orbing out, the camera is on Phoebe and on her right, you still see Paige's shoulder. And then it cuts to Paige orbing back in.
Factual error: During the ritual, we can see that the snake that is magically transformed into a man is a milk snake. But it's not poisonous, it's a colubrid. The creators apparently wanted to use its resemblance to a poisonous coral snake, which is an elapid. (00:02:35)
Continuity mistake: When Paige is at the bar with her newest charge we see the same extra walk by 3 times, going the same direction each time.
I've Got You Under My Skin - S1-E2
Revealing mistake: At the end of the episode when Phoebe and the other two are at the bar you can notice that Prue straw in her drink goes in and out of her drink a great deal.
Continuity mistake: When Prue lets Matthew out of the locket, the front view Prue has her right hand on her stomach, when the shot changes to the back view her hands are by her side, it does this multiple times.
Something Wicca This Way Goes - S7-E22
Continuity mistake: When Zankou is looking at the spell in the Book to steal TCO powers we see that it is the same spell Aunt Gail used in "How to Make a Quilt out of Americans", except that in that episode she ripped the page out of the book. It shouldn't be there for Zankou to use. (00:09:56)
Visible crew/equipment: When Piper is coming down the stairs with the staff, if you look in the top left corner of the stairs, you can see a piece of camera equipment moving.
Love Hurts - S1-E21
Continuity mistake: Leo dies and then Piper heals him. In all future episodes it's made very clear that Leo's powers can't heal the dead.
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.