Recursive Adaptation: The late 90s/early 2000s Sabrina animated series (and its versions of the cast, etc.) was adapted back into the comics, in the context of being flashback stories to Sabrina's junior high years.
When Tim, played by George Wendt of Cheers fame, walks into Hilda's coffee shop in a later episode, everyone raises their mugs and greets him, "Tim!".
Aunt Irma is played by Barbara Eden, who also played in another supernatural sitcom. Her magical gesture in both? Crossing her arms.
Robin Ricker plays Cousin Marigold, the mother of a bratty witch daughter. Around the same time she also starred in the Buffy the Vampire Slayer episode "Witch", where she also played the mother of a witch daughter or so we think as the episode goes on. Buffy even says during the episode "she's our Sabrina".
David Lascher previously played a character called Josh in Clueless - which had a crossover with this, featuring Sabrina.
This is not the first time Phil Fondacaro has played a troll.
Awesome, Dear Boy: Soleil Moon Frye and Melissa Joan Hart had been good friends since their child star days and had longed to star in a project together.
Channel Hop: Moved from ABC to The WB. As a result, most of the cast members aside from the Spellmans, Salem and Josh were fired (though Harvey was brought back eventually after fan protests).
Dawson Casting: Pretty much the whole "teenage" cast but notably Melissa Joan Hart (Sabrina) (b. 1976), Jenna Leigh Green (Libby), (b. 1974) and Lindsay Sloane (Valerie) (b. 1977). One of the student extras was a balding man!
Executive Meddling: According to Michelle Beaudoin, the reason Jenny was axed was because the network wanted Sabrina to have an 'ethnic' best friend. This is odd, seeing as her replacement Valerie wasn't ethnic (unless being half Jewish counts). Sabrina does gain an ethnic best friend in Valerie's replacement Dreama.
The Other Darrin: Similarly to M*A*S*H, very few actors carried over from the original Pilot Movie to the series proper. Other than Melissa Joan Hart, only Michelle Beaudoin, who played Marni in the movie and Sabrina's (first) best friend, Jenny, in the series, appeared in both of them. Hilda, Zelda, Harvey, and the voice of Salem were all re-cast. Libby does not appear in the pilot; a character named Katie fills a similar role. There's also a character named Seth in the movie who acts as Wrong Guy First; he has no analogue in the series, as Sabrina is hot for Harvey (which is more or less requited) from the outset.
A more conventional example, during the show's run: the two actors who played Sabrina's father not only looked totally different but the replacement was visibly years older.
Cousin Marigold is another example, first played by Robin Ricker and later Hallie Todd.
Real-Life Relative: Recurring character Amanda (Sabrina's evil cousin) was played by Melissa's little sister, Emily Hart. Who, as noted above, ended up playing Sabrina herself in the cartoon. Likewise Amanda's little sister Allie is played by another Hart sibling - Alexandra Hart-Gilliams.
Shout-Out: This show continues the tradition begun with the comic series and perpetuated by Bewitched of giving most female witches a name ending in the letter "a"—Sabrina, Hilda, Zelda, Vesta, Lydia, Amanda, Sophia...
They attempted two spin-offs with Amanda. The first would have been about her, Allie and Marigold moving in with Emile and his boys - one of whom played by Frankie Muniz. The second would have focused on her hijinx at Witchright Hall. Neither happened, though Emily Hart did get to voice Sabrina in the Animated Adaptation.
The plot of Season 7 would have revolved around Josh proposing to Sabrina and the growing love triangle with Harvey. When David Lascher opted to leave the show, Aaron Jacobs was created to fill his role. Additionally they wanted a two-part finale but were only granted one episode.
Some plans for a Season 8 would have involved Sabrina and Harvey moving into a haunted house. Barbara Eden would have joined the show as a regular.