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.
Answer: "Hush, Hush, Hush" Paula Cole.