Pre-Witched - S3-E17
Revealing mistake: At the beginning of the episode Piper and Leo are in the shower together - at the bottom of the screen you can see Piper has on a flesh colored towel.
Other mistake: At the start of the episode when Prue and Phoebe are walking down the street (just before they find the box), the person with the green jacket on (in the background) reaches into the homeless man's hat and just "scrapes" the bottom of it. Not putting any money in, or taking any money out - a completely pointless action. (00:02:05)
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)
Continuity mistake: When Prue talks to Piper after stopping her phone conversation, she tells her that "the Sin Balls work a lot faster than [they] thought. Once infected, the victim only has a few hours before it drives them to total self-destruction." However, this is almost verbatim what was written in the Book of Shadows. It could be easily read by the viewing audience when the book entry was shown. So all three sisters should already know this.
Continuity mistake: When Prue telekinetically moves the sin ball box across the street she moves it ALL the way across but when the man infected with greed goes to get the box, the box is only halfway across the street.
Continuity mistake: When Prue and Piper leave to help the hostage, Piper asks Leo, "Do you think she's mad?" Her hair is in front of her shoulders. Immediately afterwards, she sits on the couch with Leo. Her hair is behind her shoulders.
Continuity mistake: During the scene where Piper is ordering merchandise from Bloomingdale's and Prue, she and Phoebe realize the "sin balls" infected them. Piper's hair constantly changes from being in front of her shoulders and behind them.

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 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: 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)

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: Piper says she wants to go to Paris by plane not by orbing, but in the episode "From Fear to Eternity" flying was Piper's greatest fear.
Revealing mistake: The sound of Phoebe writing with a permanent marker was added in, as you can see that the marker never touches the paper. Then after a few seconds Phoebe stops writing to listen to Prue and her marker isn't moving but the sound of writing can be heard. (00:05:30)
Continuity mistake: When Phoebe and Prue look in the Book of Shadows for the spell to summon Belthazor, it is written with the same artistic calligraphy as the rest of the Book. But this spell was written by the sisters themselves in episode 3-8: "Sleuthing With the Enemy." As a result, it should have a handwritten appearance similar to the pages Phoebe was working on at the beginning of this episode.
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.
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.
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)
All Hell Breaks Loose - S3-E22
Continuity mistake: When they first bring the doctor to the manor the sisters tell him they're not sure who's trying to kill him, then a few minutes later Prue tells him exactly whose trying to kill him, Shax, the Source's assassin.
Answer: "Hush, Hush, Hush" Paula Cole.