In the Charmed episode "All Halliwell's Eve", Phoebe gets told that her true love's name will start with a C. At the time, Phoebe thinks it refers to Cole. However at the end of the series Phoebe marries Coop whose name also starts with a C. It could also be a reference to her boyfriend Clay from New York before she became a witch.
Further. Clay, Cole, and Coop were arguably her most influential romances (Clay before the series, Cole in the middle, Coop at/after the end). There are three of them. C is the third letter of the alphabet. Phoebe is the third of the Charmed sisters. Is anybody else seeing a pattern here?
Not quite sure if this is best classified as "Fridge Horror", but it certainly gave this troper some: the scene in "The Power of Three Blondes" where the 3 titular blondes magically usurp the sisters' identities and then start coming on to Chris. Towards the end of the episode, one of the fakes asks her sister, the one who's stolen Phoebe's identity, if she slept with Chris; he says "yes". This despite the fact that as far as under-the-spell Chris is concerned, the woman in question is his BIOLOGICAL AUNT WHOSE CARE HE GREW UP UNDER. A quite credible argument for the idea that his relationship to the Charmed Ones wasn't conceived by the writers until after the start of the season it happened in.
Or he was lying- he was trying to create discord at the time, remember.
Earlier in the episode, Chris acts a little disturbed when Fake!Phoebe is hitting on him, (this is before he figures out that she's an imposter) and even firmly tells her that "I'm really, REALLY not interested." He was almost certainly lying, though. Note that Fake!Phoebe denies sleeping with him, and given that the other Stillman sisters believed Chris without any evidence in spite of their sister's denial, there wasn't really any reason for him to actually sleep with Fake!Phoebe.
Billie is seen as being some sort of a Mary Sue because of the show's focus on her story as well as the many feats seen on her part which were never hinted at being feasible at least by any non-Halliwells before. But aside from the side story focus not being terribly much greater with her than it was with the likes of, say, Cole, she and Christy's story has involved a terrible lot of calling out the Charmed Ones over shirking their duties for their personal lives even as far back as that being what convinced them to drop their false faces and lives. Thus, it all makes sense. Aside from Kaley Cuoco portraying her like this was one of her sitcom characters half the time (which she can't really be faulted for as it's the ones behind the scenes who cast her in this role yet didn't write to her strengths), everything that makes Billie appear Sue-ish, from her outdoing the Charmed ones at demon hunting early on to her and Christy suddenly being the Ultimate Power and almost matching them, wasn't because of how she was this great, nigh-unstoppable prodigal super witch. It was her determination to do good and hunt down demons, especially due to her sister's kidnapping, plus the Triad's manipulation of everyone involved, versus the sisters' increasingly single-minded determination to fulfill their dreams getting in the way of their Wiccan call of duty (to the point, as mentioned above, where they changed face hoping to completely skip out on their world-saving job). The Charmed Ones' experience with Billie was a cautionary tale about the consequences of neglecting duty.
The comics revealed that Prue's presence was holding the Power of Three back during the Ultimate Power battle as well. In other words, one former God-Mode Sue practically just helped another be one. Destiny must be an even more powerful force than believed…
When Wyatt is evil in the original future Chris came from, he speaks with a vaguely British accent that he doesn't use when he is good. Why does he use it? Because he turned evil because Gideonkidnapped him as a child. He subconsciously absorbed that this was what evil sounds like.
In 03x01, Cole tells the demonic judge, 'Free Willy' he'll "take it from here" before promptly vanquishing him. Later on, we find out he's here to destroy the Charmed Ones like Free Willy was.
In the starting scene of "Sympathy for the Demon" Phoebe (now infamously) misspells her own name. That could be now seen as a hint that reality wasn't what it appeared to be.
Actually, Phoebe misspelled her name on the real divorce documents (which would probably render them null and void) when she signed them at the end of "A Witch's Tale, Part Two". The fact that the divorce papers needed to be signed again was the hint that Cole was hallucinating.
It seems a bit strange that it took so long for Paige's Healing Hands to kick in, especially since the trigger is love and it's not like no one she loves was ever seriously wounded, particularly during seasons six or seven, where you would expect her to be far enough along in her power growth to be able to access her last Whitelighter ability. However, when Piper switched powers with Leo in order to heal him during the first Darklighter episode, she is unable to do anything until she admits that she loves him. Paige's healing ability doesn't kick in until Henry is dying in her arms and she has no way to save him. Judging by the look on her face, it seems pretty clear that, if she didn't suspect it before, she knew right then that she loved him. So, basically, it seems that if a person isn't a full Whitelighter, they don't have the same strength of love in them to use the Whitelighter healing power until they've found the love of their life and admitted it, either out loud or just to themselves. (Note that Piper and Leo's son Wyatt, who is a Half-Human Hybrid of the exact same vein as Paige, can use the healing ability at birth, however as he's The Chosen One and can do a lot of things that normal witches, hybrid or not, should be unable to do, he's not really a fair factor.)
Also before when she first tried healing. In "Charmed & Dangerous" she was able to heal Leo by tapping into his powers as well. She did love Leo somewhat at that point so that probably helped her do it then.
A meta example. The third season finally features Prue being called out on a lot of her Mary Sue tendencies, as well as not dominating the majority of storylines. Creator Constance M Burge left the show after season 2 (though stayed as Executive Consultant until season 5) and Prue was her Author Avatar. It makes sense the character wouldn't be as central.
The sin episode. Piper gets infected with Gluttony and instead of overeating, she demonstrates more of the original Biblical definition of the sin; she overindulges by buying excessive ornaments, clothes and jewellery without a thought to her finances. It's also further separated from Greed by having Piper order things like champagne and having flowers delivered to herself.
Before the hostage situation and Prue discovers that she's infected with Pride, there are subtle hints to it earlier on. She lectures Piper about her finances - wanting to show off her maturity and sense. She also doesn't object to Piper not coming along - giving herself more of a chance to show off her selflessness. She also immediately assumes that she wasn't infected.
Billie MacGuyvering up a potion with normal ingredients. It could be a manifestation of her projection power, willing the stuff to work magically.
Piper's power initially being classed as 'freezing time' and then later to freezing molecules. The Charmed powers were used by Melinda Warren in the 17th century. In-universe example of Science Marches On. Of course a woman in the 17th century would have no idea about molecules and would assume she was stopping time. This reveal also explains why Piper is able to selectively freeze things - along with being able to only freeze certain parts of someone's body.
Why does Paige get telekinesis as her powers? Because there are only three Charmed powers and with multiple children they seem to go - telekinesis, freezing, premonitions. She was born when the three other powers had already been distributed, so it was just starting from the beginning again. If there had been another sister, she would have gotten freezing.
Patty's theory about substituting the Power of Three in "That 70s Episode". Just after Phoebe was first conceived, Piper and Leo came from the future to have them recreate the Power of Three (with Penny from further on in time) to vanquish the Hollow for good. Although she was returned to her time to just before she was taken (erasing her memory of it) perhaps she still had some deja vu about it - which is where she must have gotten the idea from.
In "Charmed & Dangerous" the Seer's line "I didn't do this for your future. I did it for mine" seems like she just helped them to secure a future for all people - mortals and demons. However as the season progresses we learn that the Seer's master plan was to impregnate Phoebe with the Source's child and take control of the baby to inherit the powers.
How do the Charmed Ones suddenly have a lot of knowledge about upper-level demons in the Book of Shadows? Well they do have one living in the house in Season 4. It's possible a lot of the entries in the Book of Shadows come from what Cole knows. He's bound to know what a good few of them look like - their strengths and weaknesses etc.
Charmed has a particularly dark case involving Penny (the Charmed Ones' grandmother) dying. Before she died, she had made a potion to strip the sisters of their powers permanently. It was quite lucky she died before she gave it to them, wasn't it? Almost as if some higher power had a hand in her death such as the Elders maybe? Given that Penny was going to de-power the most powerful good witches in the world, it's almost definite the Elders stepped in and killed her before she got there.
Speaking of the Elders, after Gideon's assistant found out about Gideon's evil plan, Gideon blasted him into a pile of ashes to keep him from talking. Bad enough on its own, right? But this happened at Magic School, which is magically enchanted so that nobody can die there. Even if you're beheaded, you still remain conscious and aware as long as any part of your body remains on the school's grounds. So as long as even one speck of that guy remains embedded in the carpet... yikes.
An in-universe example in "Be Careful What You Witch For". Piper and Phoebe recall a story in which an abusive boyfriend attacked Prue and how the situation enraged Grams. Piper notes the boyfriend disappeared not too long afterwards. In the spirit of the trope, something that she didn't know then but now does (Grams being a witch) makes Piper consider there was a supernatural reason for his disappearance.
In Season 5's "Sense and Sense Ability," the Crone foresees Wyatt's future and is downright terrified by it. ("I've seen everything!") It seems like it's just because Wyatt is a Halliwell - a de-facto force for good and a powerful one at that. But then Season 6 reveals that (before Future Chris' time-travel trip) Wyatt grows up to be a God-Mode Sue that ruins the world.
In the Book of Shadows, Shax is described as the Source's personal assassin and is assigned only for the top assignments. I highly doubt killing a mortal paragon of good counts as a top priority and sending Shax could be considered over kill. Now killing the Charmed Ones is a different matter. The Source's real goal was to send Shax after the doctor and, knowing the Charmed Ones, have them go to rescue him and get them killed in the cross fire. He could have even have manipulated Phoebe's vision (that she mentioned early in the episode) to ensure they would be involved.
Fridge Brilliance: The Seer has shown she's able to manipulate Phoebe's premonitions. Perhaps she had a hand in it?
See how in love Penny and Alan are in "Witchstock". After his death it's established he became The Lost Lenore and Penny became the famous demon hunter. However remember that losing Alan caused her to become bitter and cynical towards men - having five more failed engagements and four failed marriages. She ultimately died alone and still bitter over losing Alan.
The numerous times Prue gets lectured about not diving so heavily into her witch duties. She really should have listened shouldn't she?
The premonition of all the deaths Phoebe gets in "Charmageddon". Two of them are ones she didn't already see - Penny and Chris. So in addition to having to relive the other deaths, she also had to see her grandmother and nephew die for the first time.
The Book of Shadows contains descriptions of many unique demons and warlocks, often together with a spell and/or potion to vanquish them. Which raises the questions:
a) Why they haven't been vanquished by previous Halliwell witches? b) How do they know that said spell/potion will actually work?
Probably the biggest excuse through the course of the show: The Power of Three. The spells worked to wound or weaken the demon, but the Power of Three worked as something of an amplifier to push the power of the spell or potion over the top. Many spells were described as a "Power of Three Spell," which makes them sound like only a trio of witches could do them effectively.
Also, perhaps one of their ancestors looked into the future and took notes?
Key word here is "vanquish", which mean "To defeat or conquer in battle; subjugate." Not to outright destroy them, but simply to defeat them. Some of the so called vanquished demons return frequently (Cole, Barbas). And at least one of them is explicitly said that has not recovered from the last vanquish he suffered (Tempus, in the Season 3 finale).
Is it just me, or does Piper seem to forget that she has the power to freeze after she fully develops her exploding power? I understand that it's easier to just blow up a weak demon on the spot, but from season 4 onwards, she hardly ever freezes anything, even when it would be extremely useful. I remember several occasions when she was fighting demons/other creatures strong enough that her power of combustion couldn't fully blow them up, only annoy them or slow them down, and she just continues throwing small explosions at them to slow them down instead of freezing them, which would be much easier and smarter. Could be justified if they were also strong enough to resist her freezing power (as several strong demons have been known to do) but she doesn't even try it. Forgotten Phlebotinum perhaps?
Upper level demons were more resistant to her freezing power. There were some demons it didn't work on at all. However they can't be immune to her explosion power. It was a better strategy trying an offensive attack that had more of a chance of working than trying a power that they might be resistant to.
A minor quibble with "Centennial Charmed". If Cole lost his Avatar powers when he created the alternate reality, when he was vanquished and his spell was reversed, shouldn't he have immediately regained his Avatar powers once more, thus been able to reassemble himself from nothing?
He shouldn't have even lost his Avatar powers in the first place. If he was limited to just Belthazor's powers in the alternate timeline, he wouldn't have had anywhere near the power to maintain such a powerful spell, and reality should have snapped back to normal immediately.
During Leo's crusadeagainst evil during Season 7 at no point does it occur to anyone to point out that Wyatt seeing his severely pissed off father solve problems by simple blasting them with lightning, might be the very thing that caused him to turn to evil in the first place. After all, they only assume that Gideon's betrayal was the sole event that turned him to evil and made him a sociopath.
Furthermore, we later see even minor events such as his teddy bear being cursed are capable of shifting his morality, to the point where even Vicus lampshades how quickly he fell to evil. As demonstrated several times over the course of the series, the future tends to always be in flux, so why do they continually assume that Good Wyatt is permanently here to stay?
The long-term thing is key here. Wyatt turned evil because he was forced to defend himself against Gideon's attacks for months. It's likely he was forced to kill Gideon himself too. In the new timeline he may have seen Leo kill Gideon but he was still surrounded by a loving family almost immediately after. And they could have removed his memories of seeing the event if they were that worried. For the teddy bear, the thing here was that the demon slowly corrupted him. Future Wyatt turned evil because the future had been changed to if he had grown up with the demon in the toy whispering in his ear.