Other mistake: After Sam uses the powder on the sisters, erasing their memories, they're visited by Leo who starts to say "he must have gotten the powder" and one of the sisters finishes with "our mother." But in a later episode we learn all white lighters have this powder.
Charmed (1998)
2 other mistakes in P3 H2O - chronological order
The Importance of Being Phoebe - S5-E11
Continuity mistake: When Phoebe is lying on the ground and is supposed to be unconscious, when Cole goes to pick her up you can see Phoebe throw her arms around his neck. (00:29:15)
Cole: Phoebe, I love you. I don't know what's going on but maybe I can help. Would you like me to kill someone for you?
Trivia: Many actors who have made appearances on Charmed have also made appearances in Buffy. Some are Greg Vaughn, Charlie Weber, Wade Andrew Williams, Kevin Weisman, Christopher Wiehl, Daniel Dae Kim, Michael Bailey Smith and Brian Thompson and Charisma Carpenter.
More questions & answers from Charmed
Answer: "Hush, Hush, Hush" Paula Cole.