Hulkus Pocus - S8-E9
Other mistake: When Piper is explaining to Phoebe that they don't have enough time to save Billie, she lists all the things they need to do ending with "fix us and then fix Billie..." This doesn't make any sense, since when she's saying it there's nothing wrong with them to fix. It's only after she says this that they come up with the plan to infect themselves. (00:29:54)
Answer: "Hush, Hush, Hush" Paula Cole.