Plot hole: One of the sisters asks why the familiar doesn't just jump in front of a bus 5 times, another responds with "we're the only ones strong enough to kill him." If they are the only ones strong enough to kill him, then how come two scenes later he falls on a knife during a fight with Phoebe and dies? If a knife can kill him then it doesn't make sense that he seeks out the charmed ones.
Other mistake: During a flash back of Grams in the kitchen she puts a rose in a pot, then Piper walks in, they talk and Piper starts to add some salt and a couple other ingredients into the pot. Grams asks how she knows what to add, Piper says says something about mustard - Piper wouldn't know what's in it already, she just walked in. And she never even finds it strange that there's a rose with a stem in it.
Continuity mistake: Kit the cat shows up in Something Wicca this Way Comes (Episode 1,1), as the girls' familiar once they have received their powers. It is obvious that the cat just showed up by comments made by the girls, along the lines of "where did this cat come from" (not literally). However, in Pre-Witched (Episode 3,17), they make the same kinds of comments about Kit just showing up in this episode, which is suppose to have happened 6 months before Episode 1,1.