Pre-Witched - S3-E17
Corrected entry: 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 supposed to have happened 6 months before Episode 1,1.
Correction: It's 6 months apart. A cat showing up once and then again 6 months later isn't cause for mistake. Simply explained through forgetting.