The Charmed Ones and the Elders struggling to figure out what demon killed Marisol and the other Elders makes much more sense when one considers that it was actually another witch (another Elder, for that matter) who killed them - they were so focused on looking for a demon that never existed that they missed the signs that it wasn't demonic.
Mel being the sister to try and recreate the Book of Shadows after its destruction in the first episode of season 2 makes sense, as she was the sister who was the most willing to become a witch in the first episode of season 1, so she would have therefore looked through and memorised the contents of the Book of Shadows much more than Maggie or Macy. Plus, after they rewrote history and Mel never got her job as a Teaching Assistant, she had more free time to look through the book than Maggie and Macy, who both had jobs and, in Maggie's case, college commitment.
In Season 3, the Charmed Ones have to find the hidden pieces of the Source, which they each hid in a separate location. Mel and Macy hid theirs away from the house, but Maggie just hid hers inside one of her own possessions, which may seem weak, but makes sense when considering what their powers at the time were. Macy, as the 'Sister of Space' with the power of telekinesis, hides her piece on a remote island on the other side of the world, a location that is spatially distant from her; Mel, as the 'Sister of Time' with the power to manipulate time, hides her piece in the future, a location that is temporally distant from her; Maggie, as the 'Sister of Emotion' with the power of Empathy, hides her piece in a secret compartment in a music box she believed her estranged father had given her, a location that, at the time, was emotionally distant from her. Space, Time, and Emotion.
When Fiona and Charity talk about the fact that Alastair didn’t consider that Charity was still an Elder, Fiona mentioned that she had another vessel for the Source in mind. When she was looking for the origin dagger, and considering that she had Harry with her when she looked for it, it might be possible that she planed to use him as a vessel, and kill him which is horrible because of the fact how much he cared for her and how her alleged death destroyed him.
In an episode of season 2, Harry and Abigael encounter a young girl named Cassie who can freeze people telepathically, but hasn't learnt how to control her powers and accidentally froze an entire town and was unable to unfreeze them until Harry taught her how. What would have happened had Harry never found her? And how many other areas have had similar circumstances?
