On a related note: In one episode, Damon walks in on Stefan and Elena after they've had sex. Both complain, having not gotten a chance to rest yet, to which he just says, "If I see anything I haven't seen before, I'll throw a dollar at it." At first, this looks like a nod to his ladykiller past/present, but then you pause for a moment. Stefan is his brother, which makes avoiding at least glimpses all but impossible, and Elena is literally a doppleganger of his ex-lover. Yes, he has genuinely seen it before. —Red Wren
When Tyler's car crashes due to the effects of the Gilbert Device, his eyes turn golden. Of course this is to foreshadow that he has the werewolf gene, but it makes no sense as one has to kill someone (accident or not) to activate the curse. Then it hit this troper that Caroline was at that point dying of internal bleeding caused by the crash; in other words the curse was just on the edge of being fulfilled.
Or it just affects everybody that has the werewolf gene whether it's been activated or not, like with Tyler's father.
Seeing as we aren't getting a Christmas hiatus in Season 4 we have a higher chance of getting a holiday episode. Of course its happiness level might be lower than the one Veronica Mars had.
Damon being the one to realize the sire bond in Tyler makes a lot more sense in Season 4, when it's revealed that he had a vampire sired to him in 1940s.
Stefan seemingly enters Caroline's house without an invitation in "Bad Moon Rising", but in "The Turning Point", Stefan rescues her from Logan and probably was invited in.
Katherine says that she had been taking vervain for 145 years in "Memory Lane". It seems to be just so she's not harmed by it, but in "Katerina" we learn that Originals can compel other vampires, so it also had the added bonus of protecting her. This might even have been the main reason, considering she's spent her entire life as a vampire on the run from Klaus, and naturally would want to be protected from compulsion.
In "History Repeating", Damon vows to drink animal blood, but nothing with feathers. Though seems like a simple joke, if you remember in "You're Undead to Me", Damon killed the crow that followed him around for sustenance. He had a good reason for not wanting anything feathered.
We also learn that in Season 7, Damon had a pet turkey as a child, which his father made him kill for Thanksgiving dinner on the grounds that "that animal was put on that Earth for sustenance." Damon might not want to drink animal blood from animals with feathers, because of that.
In "We'll Always Have Bourbon Street", Damon meets a witch that doesn't practice anymore. In The Originals, we find out just why she doesn't practice anymore. It's punishable by death
Caroline's seeming ease controlling herself and her murderous urges once she becomes a vampire seems MarySueish at first, but it becomes brilliant when one remembers that becoming a vampire turned her into, as she put it, "an insecure, neurotic control freak on crack". Basically she has a predisposition toward control including of herself that got turned Up to Eleven when she became a vampire.
While Damon doesn't get along with Caroline, he hated her father for, among other reasons, torturing her by taking her daylight ring and burning her with sunlight. This becomes much clearer in "Mommie Dearest", where it is revealed that his own father burned him with cigars.
In season 7, Damon has been hit hard with a bad case of Informed Wrongness, with everyone blaming him for Rayna going after Stefan, when in fact it was Enzo's fault for manipulating events. However, since Damon turned all of their lives upside down with his return to Mystic Falls in season 1, it makes sense that their first instinct is to blame him when things go wrong.
But they were also angry with him for abandoning them when he desiccated himself and intended to sleep until Elena woke up so that they could be together again and the three years he was "asleep" having to not just deal with Rayna but other incidents he could have helped them with. Even then Stefan and Bonnie were the most hurt and upset by his decision to do this because for one thing Bonnie would have been dead when Elena woke up and she would have never seen Damon again and they just became best friends and Stefan because of the fact that his brother is abandoning him just when he needs him. Also Damon's decision was also motivated by his desire to not hurt his brother and his friends anymore because of his bad habit of doing terrible things to them and other people because of his vampire nature and they were also angry at him because of the fact he was saying goodbye to them through letters.
In season 8, Sybil is able to control Damon far easier than Enzo due to the fact that Damon turned off his humanity. Without it, he is less likely to feel his emotions, and therefore is less likely to fight for the things and people that he cares about. This actually ends up helping Sybil become irresistible to him as a result, as she is able to insert herself into the various empty places.
Also, keep in mind that Damon has just spent most of the previous season being guilt-tripped by everyone over everything he has ever done, and probably still had his issues in his head when Sybil got to him. So, he turned off his humanity as to not feel any additional guilt, leaving him susceptible to Sybil. Other than Elena (who is stuck in a coma thanks to his mother), he probably felt that he was just a disappointment to everyone else and wasn't worth saving. Enzo, on the other hand, just spent the past few years getting his confidence and self-worth rebuilt thanks to his relationship with Bonnie, and felt that he still had something to fight for.
Sybil hasn't bothered to have Damon go after Bonnie again like last time because after she was captured by the others, she was there when Stefan and Bonnie talked about saving Damon vs saving Enzo. Based on Bonnie's words, she must have concluded that Bonnie has washed her hands of Damon. Therefore, Bonnie is no longer a threat to her control.
Maybe she has washed her hands of him, considering that she is the only one who hasn't dealt with him since.
At first, I thought that vampire blood speeding up cancer was a forced way to kill off Liz Forbes. But, once I started to think about it, it all made sense. Vampire blood is used to heal things, usually wounds, and the one way all things heal is by having the cells divide until it's healed. With cancer, it's a disease that's just that: rapid cell division. The vampire blood would of course speed up an already sped up disease.
Unlike in most media where vampires possess mind control powers, compulsion does not seem to particularly affect the mental faculties of those it controls unless the vampire using it specifies that the victim must not remember something or should think something in particular. This is later proven in "The Ties That Bind" when one of Klaus's Hybrids compels Jamie to shoot anyone who arrives at the house; while he sticks to the orders and attacks Stefan, he clearly does not understand why he is doing it. This means that others we have seen under compulsion, such as the man Isobel compelled to step in front of a car, resulting in his death were aware of what they were doing, but completely unable to control their bodies.
Which gives one an even darker perspective of Damon's "relationships" with Caroline and Andie, though it is argued that because the sex was consensual before he started compelling them that it wasn't rape and after the things he compels them about is being okay with him being a vampire and allowing him to feed from them, but never to have sex with him, yet it is shown that the moment he reveals his nature to them they immediately want to run away making it that if it wasn't for the compelling they would have decided to never sleep with him again.
Katerina's "turning" may qualify as this, and/or Fridge Tearjerker to explain: Katerina/Katherine ran-away from Klaus when Elijah told her about her about their plans to sacrifice her, and Rose walks in on her attempting suicide ( "I'd rather die than go back to Klaus ) and forces her blood into Katherine's system. When Trevor comes home, Rose ambushes him and scolds him for what he's done, and when they turn around to check on Katherine, she's hung herself. At first, one assumes that Katherine purposefully did this to turn herself into a vampire, but then you realize that Katerina had absolutely no way of knowing this. as she learned about Klaus (and, subsequently, vampires) from Elijah, but it's highly doubtful that he told her all about turning and all that. Even though Katherine tells Elena that she had planned to become a vampire all along, all facts point to Katerina really having tried to kill herself, and her saying things like "You would have helped me run. That was never going to be enough" sounds like someone who had, originally, given up, and her saying "Better you die than I" sounds like someone who has been given a second chance at life. [[Tearjerker Which makes her personality change after turning that much more understandable]]
The show has recently revealed that blood sharing between vampires is considered very intimate with Julie Plec herself saying it's basically "vampire naked cuddling". Keeping this in mind, think back to Mikael's diet. It adds a whole new layer to the various vampires' obvious disgust towards it. It also adds to the reason why Damon was so opposed to Mikael feeding off him and why he had such a negative reaction to him feeding off Stefan. The guy just naked vampire cuddled his unconsenting younger brother right in front of him. And he did the same thing to Katherine as well.
When Elena and Jeremy kill Kol, Klaus angrily threatens to burn their house down. He's stopped from doing so, but I realized later this happens anyway, only with Elena doing the burning.
Every supernatural death goes through the anchor. Amara was the anchor when Elena and Jeremy kill Kol. That created a WHOLE lot of supernatural deaths, all at once. Though she was asleep when that happened so she probably didn't feel it or it was less painful.
Though the scary thing is how many of those vampires were good people or like Stefan made it a thing to not kill humans and its clear that Elena and Jeremy may have considered that and it becomes more harsher when there turns out to be only enough of the cure to cure one vampire meaning that they just killed thousands for the sake of one.
After Elena forces Katherine to take the cure, her body begins to catch up with all the years that it has been supernaturally alive, causing her body to start to age and decay. Katherine took the cure unwillingly, but remember who REALLY wanted it? Rebekah! If Rebakah had gotten her wish, imagine the fallout. Not only would she probably have died even faster, being more than twice Katherine's age, but the reactions of her brother's could have been... messy. Klaus is unstable enough with his siblings at least trying to keep him in check, and Elijah would have had the additional guilt of being the one to give her the cure.
Katherine only started aging after Silas drank her blood to get the cure for himself. It's a bit murky on the show, but I think we're supposed to assume that as long as the cure isn't passed on to someone else, rapid aging doesn't come along with it. Silas may have started aging if he'd lived much longer after Amara took the cure from him, but he wasn't technically a vampire and the cure was specifically formulated for his type of immortality, so it's possibly a side-effect only for former vampires who take the cure.
This becomes a factor in season 6 when Bonnie gets another vile of the cure from the witches' prison world and gives it to Damon to give to Elena and Damon makes the decision that he wants to be cured with her. Their plan was for Elena to take the cure and he would drink her blood and it would have worked since Elena was only a vampire for about 1-2 years, but if Damon took the cure first and drained Elena, he would have went through the rabid aging process since he is centuries old.
There's a scene at the end of "Graduation" where Silas takes on Elena's form. We later learn that his lover, Amara, was the Original doppelganger making the whole thing very squicky.
Every single werewolf has killed someone. At best, they have crippling guilt about the person they killed. At worst, they're cold blooded murderers.
In season seven, Matt got Rayna to go after Stefan because of a misunderstanding involving a mindwipe. One can only imagine how many more of those have occurred because of all of the mindwipes and compulsions done by vampires over the years.
Stefan's explanation of why Elena looks like Katherine. Apparently, he was only surprised by them having different last names, which could be easily accounted for by a female descendant of Katherine marrying a Gilbert, and not by the fact that people don't tend to look like their 150-years-ago ancestor's identical twin.
For the record Katherine is over 518 years old being turned in 1492. Before she was turned Katherine had an illegitimate daughter. It is highly likely that Elena is the descendant of that daughter so Katherine is Elena's perhaps 22 times great grandmother (assuming each generation is 21 years going back over 500 years).
In "A Few Good Men" Sheriff Forbes does a background check on Alaric and he has a clean record. However, in "1912" Meredith does another background check and finds several violent instances. How would the local law enforcement miss such things?
How did Elijah not realize that Elena was wearing his mother's necklace? The only way it could possibly make sense if he didn't know of it, noticed it but chose not to comment, or his memory was fuzzy after a thousand years.
In "Isobel", Isobel says that she wishes her plan to plan to get Elena to stop looking for her had been more effective...except it was effective. Elena did stop looking. It was Alaric that didn't.
In season five we meet Nadia, a vampire who is revealed to be the daughter that Katherine was forced to give up. Nadia has a lot of anger towards Katherine because she feels that her mother abandoned her when she became a vampire. This is all well and good except for the fact that Elena exists, which means that Nadia had a child who she abandoned to become a vampire and search for Katherine- doing the exact thing to her child that she is so mad at Katherine for doing to her.
Though at the the same time this might not be the case since it is later confirmed that Doppelgangers aren't related by blood as there was one before Katherine that died without ever having children and Stefan finding one of himself who lives in another state and works as an ambulance driver who is not related to his family. Basically as Silas states it is a magical occurrence and that doppelgangers are just shadow versions of him and Amara.
The Travelers cannot gather in one place without disaster striking. They become Passengers in other people's bodies to get around this, so they're not physically together. Yet, their own unconscious bodies are kept in the same cave. Together. Hmmmm.