WMG / Sabrina the Teenage Witch

The Pilot is simply Non-Canon.
So much ended up being retconned in the actual show:
  • Probably the most infamous, Sabrina was told that she could not see her mother until she obtains her Witches' license. For the rest of the series, it is made clear Sabrina cannot see her mother ever again.
    • This was reffered to repeatedly up until the point when she actually gets her witches license. And before that I believe they'd even specified it as being a time limit of a couple years so the witches license thing was already a retcon.
  • Word of God later confirms that after Sabrina's common-law wedding to Harvey - which can be seen in a file on the X-Files Website - Sabrina was allowed to see her mother again whenever she liked.
  • Sabrina was told a witch only comes into her powers (and is told about magic) when she is sixteen. Child-witches can be seen throughout the series (such as, but not limited to, Cousin Amanda)
    • Is that for all witches or only half-witches? Remember, Sabrina's mother was mortal. I'm pretty sure Amanda is full witch, so the rules may be different.
  • Sabrina's father can talk to her through a book, and tells her he will always be there for here (in the book) if she needs him. This idea is never used throughout the rest of the series, even though her father does appear in later episodes (though in person).
    • The spin-off books, ironically, actually used this idea more often, though they were a different continuity than the show.
  • To a lesser extent, Hilda making a catty remark about Sabrina's mother, since there does not seem to be any animosity between them for the rest of the series, though to be fair, it is not like they really interacted.
    • Maybe it was animosity left over from the time Sabrina's parents divorced? You could see Hilda vehemently taking her brother's side in a matrimonial falling out - and Zelda having to dissuade her from magically interfering. Hilda might have been still nursing a grudge. Hilda and Sabrina's mother don't meet again until the sixth season, by which time Sabrina's father had remarried and Sabrina's mother had been temporarily turned into a ball of wax for the "crime" of seeing her daughter. Under the circumstances, it's safe to say Hilda relented.

The entire show is a giant metaphor for the Wiccan rule of three.
The practicality of having magic in this show and using it for personal use is wildly marred by all the problems it causes. Although most of the time it is put down to Sabrina being an inexperienced witch, even her Aunts are tortured with various magical problems in nearly every episode (knocked out by magic poppies, sealed in their own home, various misfortunes because of chain letters). The Wiccan rule of three is that anything you give back comes back times 3. It seems in this show the only appropriate way to use magic is only when strictly necessary -think Tara from Buffy the Vampire Slayer-, or face horribly inconvenient consequences. Goodness knows what type of hell she'd go through if she used magic properly like we all would (improving our appearance, improving grades, zapping ourselves everywhere instead of travelling).
  • This is partially Jossed by the fact that Sabrina still suffers consequences even when she was only trying to help people. Using her powers for small, hedonistic things seem to be the only things that do not cause Sabrina problems.

Morgan was infected by an Airhead Virus at some point.
In 'Deliver Us From Email' Sabrina gets infected by an Airhead Virus and ends up acting more than a little like Morgan (the show even Lampshades it). Maybe Morgan had a run in with the same virus at some point, especially if she ever borrowed Sabrina's computer? This would explain why even though she always had a ditzy side her IQ seemed to drop later on.

Sabrina lost her virginity to Harvey
If we assume Sabrina was not a virgin when she intended to be married at the end of season 7 - which seems likely, else it would have been discussed, surely? - then Harvey seems the likely candidate. None of her later boyfriends had nearly as big a (literal) space in her heart, not even Josh who is the other likely choice.

Witches and Warlocks are all Time Lords
We've seen many occasions on Doctor Who of sufficiently advanced technology imitating the supernatural. Witches are stated to be very long-lived; Hilda and Zelda talk about their firsthand experience of eras beyond their own stated lifetimes. And come on, the Spellman family's closet is a TARDIS!!

The Fates plan all along was to get Sabrina and Harvey back together
Aaron's moving to Los Angeles was a move to push up the wedding date and the proposal, so Sabrina and Harvey would get back together. In order for Sabrina to accept rather than just let Aaron move off to Los Angeles (like she let Josh move to Prague) they needed her to think it was their plan to stop her from having Aaron. If Sabrina says no, Sabrina may never realize that Harvey still has a major place in her heart. Sabrina and Harvey drive off at EXACTLY 12:36, which is the same time they met. That screams the work of the Fates.

Aunt Irma eventually got the rule about Mortal/Witch relationships changed... but only for Harvey and Sabrina
The only mortal she ever liked was Harvey. If Sabrina and Harvey ever had kids, Harvey would be turned into a ball of wax if he came face to face with his kids. Sabrina would have worried about this a lot before marrying Harvey, so Irma would have changed the rule... (The Witches council would have been frightened of her) but only for Harvey.

Magic is actually an actively evil force
Throughout the series individuals are constantly suffering the unintended side effects of their magic often through contrived side effects. This isn't just Sabrina which could be explained as inexperience but also Zelda and Hilda who have hundreds of years of experience. There is also a huge gap in how difficult it is to cast certain spells, some spells require specific ingredients and a specific rhyme, other just require making up a rhyme, and some are cast just by pointing. While there is little reliability to what type of spell will be which there is one exception... any time magic is being used to cause problems for others it requires nothing but pointing if even that and never seems to magically backfire. This suggests that magic is actually most effective only when seeking to harm others which could also play a role in explaining why almost all of the powerful witches in the series are evil. (the one exception Sabrina's grandmother is just really old, and thats compared to all the other witches who have been around thousands of years)
http://tvtropes.org/pmwiki/pmwiki.php/WMG/SabrinatheTeenageWitch