Phoebe gets turned into a mermaid in season 5, in a plot that heavily references The Little Mermaid. Alyssa Milano was the visual model for Ariel when she was sixteen.
Season 2's "Chick Flick" episode also has Phoebe wearing a Miami High Mermaids shirt.
Retroactively in "The Jung & The Restless" Paige wonders why she never dreams about Quentin Tarantino. One of her first post-Charmed roles was in Tarantino's Death Proof. She would indeed have a scene with the man himself.
A season 5 episode has Paige attempting to colour-code the Book of Shadows, Piper getting annoyed with her for 'defacing' it. When Rose McGowan first joined the show, she would sometimes write her lines or doodle in the prop Book of Shadows between takes - not realising how much sentimental value it had to the cast and crew.
The Danza: Alastair the chameleon demon, played by Alastair Duncan.
Dawson Casting: albeit a minor one. Phoebe and Paige are younger than Piper, but their actresses Alyssa Milano and Rose McGowan are older than her actress Holly Marie Combs.
Directed by Cast Member: Shannen Doherty directed three episodes: The season two finale "Be Careful What You Witch For", the season three episode "The Good, The Bad, and the Cursed", and that season's finale, "All Hell Breaks Loose", where she gets Killed Off for Real.
The creator, Constance M. Burge, left the show in Season 3 due to creative differences; she wanted to focus on the sisters and their relationship, whereas the producers and executives were more interested in doing more stories about their boyfriends, which is why the Cole storyline stretched on as long as it did.
Andy's death in the season 1 finale was in order to free Prue up for more storylines.
After the heavily serialized Season 4, the network asked production to make Season 5 to be more stand-alone. This was a particularly odd request as the ratings were strong and final episodes of Season 4 were highly rated. Tellingly, after Season 5, the series returned to more season-long arcs.
One of the stipulations for an eighth season was an inclusion of a younger character who could potentially star in a spinoff. The budget was also cut in half, resulting in Brian Krause having to be written out of the show temporarily.
An interesting one for the eighth season - they demanded that demons look more...well...demonic and that the vanquishes be more exciting.
Inverted in the fifth season - Rose McGowan was annoyed with Paige getting so many different guys each episode and complained to the writers. This results in long term love interests like Kyle, Richard and Henry.
Giving Alyssa Milano and Holly Marie Combs free reign on the show really mucked up a lot of the series dynamic due to the fact that they were total divas and wouldn't allow their characters from being wrong in their actions (i.e the whole "Cole can't be trusted" thing)
She's not the only one off the Normandy to show up in Charmed: Raphael Sbarge, who voices Kaidan Alenko, shows up in season one.
And on a non-Mass Effect related note, Gideon? He eventually became well remembered for a certain speech about how much he loves war.
McLeaned: Prue, due to a conflict with the actresses and several people working on the show, Alyssa Milano in particular. Notably, Doherty directed the episode in which Prue was killed off. Granted, the writers no doubt had options to bring Prue back, but by the point of filming, it was quite likely she wasn't coming back for Season 4. Or ever again, (un?)fortunately.
Missing Episode: On the first DVD of Season 6 the episode titled "Love's a Witch" was listed on the DVD menu but not actually present in the DVD collection of videos, a recall was issued shortly afterward in which you could trade in your 'bad' copy and get a 'real' copy.
Money, Dear Boy: Rose McGowan (Paige) was always eager to point out that she just liked having a steady paycheck. Although, by the time the show was canceled, she did express some bitterness at how the show was treated, so it seems that at some point she did begin to have at least some affection for the show. Similarly, she's vehemently opposed the possible reboot in development when it was announced in late 2013.
Playing Gertrude: There is a twelve year age difference between Finola Hughes and Jennifer Rhodes, who played Patty and Penny respectively. Semi justified since Rhodes initially appeared as Penny in the present (well the 90s anyway) - and other episodes had her play the character in flashbacks. When a season 6 episode had Paige going back in time to meet a younger Penny, Jennifer Rhodes was too old to believably play Penny at that age - so a younger actress was cast. Of course only ten years separated the events of that episode with a Season 1 episode set in the 70s (where Rhodes played Penny).
Eighteen years also separate James Read who plays Victor, with Shannen Doherty who plays the eldest daughter. Since Read was already grey-haired when he appeared on the show, it's believable. Finola Hughes is also only thirteen years older than Doherty but that's justified since she only played Patty before she died.
Reality Subtext: The first few episodes that featured Paige dealt with Piper and Phoebe feeling like she wasn't taking her responsibilities seriously and, specifically, disrespecting the Book of Shadows, including throwing it out the window. Apparently, something very similar happened behind the scenes. Rose McGowan, not realizing how much sentimental value the prop Book of Shadows had to the cast and crew, would actually write down her lines and doodle on the pages of the book in between takes.
Romance on the Set: While on-screen, Leo and Piper were together as were Cole and Phoebe. Off-screen, Alyssa Milano was dating Brian Krause while Shannen Doherty and Julian McMahon were a couple. Holly Marie Combs also ended up marrying one of the key grips.
Separated-at-Birth Casting: Jennifer Rhodes and Shannen Doherty definitely look like they could be grandmother and granddaughter respectively. Likewise the resemblance between Doherty and Emmalee Thompson (who plays Prue as a young child) is uncanny. There is also a big resemblance between Finola Hughes and Alyssa Milano (mother and youngest daughter respectively). This is especially notable in season 4 when the two have similar hairstyles. And rounding off the SABC, James Read as Victor has a resemblance to his middle daughter Holly Marie Combs.
Outside the Halliwell family, Kayley Cuoco has a distinctive resemblance to Barbara Niven who guest starred as Billie's mother in a couple of episodes.
What Could Have Been: Although she had difficulties with several people, the biggest reason Shannen Doherty left according to most sources was due to several disagreements with Alyssa Milano, apparently saying something to the effect of "either she goes or I go". How different would the show have been had it been Alyssa who left instead.
Actresses Tiffani Theissen and Jennifer Love Hewitt were approached to become The Other Darrin for Prue. After they both declined, producers decided to create a new character.
In late 2013, soon after news of the reboot series was announced, Alyssa Milano sent out a tweet that floated the idea to her costars of doing a Charmed reunion movie. Shannen Doherty responded in the affirmative, also retweeting a fan suggesting funding the project through Kickstarter. After the success of the Veronica Mars movie through Kickstarter, the possibility is there.
It wasn't just Alyssa, Shannen also had many problems with the producers of the show, due to creative differences; the same reason that led to creator Constance M. Burge leaving.
Written By Cast Member: Brian Krause, who plays Leo, wrote (parts of) "Sense and Sense Ability" (the one where "Paige is proposing violence against the monkey!"), inspired by a monkey prop he saw on the set.