Chloé's biggest Fatal Flaw is her apparent inability to grasp that her actions have consequences, both in the short and long-term, such as when Gabriel bans her from any contests related to his brand, only for her to express confusion when her attempt to sign up for KIDZ's Friendship Day treasure hunt was rejected. This issue can be traced directly to Mme. Bustier's teaching style; not only does Mme. Bustier constantly refuse to punish her, but she also pressures the rest of the class to include her in their activities as much as possible. Chloé honestly doesn't understand how consequences work because her overly permissive father and teacher have ensured she's never had to deal with them before.
Her complete disregard for other people's lives, as shown in "The Gamer", "Volpina", "Captain Hardrock", "Syren", and (most importantly) "Prime Queen", can also be linked with this flaw: the fact that people could greatly suffer if she doesn't act fast enough to save them never registers since, as far as she cares, sending the Miraculous Cure just magically fixes everything.
In "Captain Hardrock", Chloé shows up to the Liberty completely uninvited and is shocked when she's refused entry. This makes perfect sense when you remember Mme. Bustier makes her classmates invite her to group outings on a regular basis. She gets so outraged at Anarka because she's used to adults forcing her peers to do what she wants, not enforcing their boundaries!
Despite getting banned from any contest involving the Gabriel brand in "Mr. Pigeon", Chloé shows confusion in "Ikari Gozen" that her application to compete in the KIDZ day contest was denied. "Despair Bear" has her initially upset that Adrien ended their friendship, only to delude herself into thinking he was being overdramatic and "forgiving" him for it, acting as if they're still friends. She likely had a similar, offscreen thought process to this regarding Gabriel, Adrien's father, and thought that he was also being "dramatic" and not serious about the ban.
In "Stormy Weather", Aurore complains to Hawkmoth that Mireille "never dresses professionally," to which the villain replies "Nothing worse than a sloppy-looking crew." As a fashion designer, Gabriel would really believe that.
At the end of "Copycat", Adrien and Alya are both surprised to learn that Théo Barbot is a student at Collège Françoise Dupont. None of the other students are shocked by this since they've all been attending school with him much longer than the two newcomers.
Chloé's belief that her father is an Ultimate Authority Mayor is partly explained by his antics in "Rogercop" and "Frightningale": The mayor isn't above lying to make it seem like he has more power than he actually does and has seemingly gotten away with it more than once, leading to Chloé having an inflated sense of her father's authority.
Why did Marinette make an extra bee-themed accessory for Chloé at the end of "Vanisher" despite the enmity between her and her bully and how Chloé's only shown disdain for any hero other than Scarlet Lady? She knew she had to be prepared in case Mme. Bustier tries to force her to include Chloé, like the teacher normally does for class activities.
When answering a question about events in "Riposte", the author writes, "... the tryouts have nothing to do with who goes to school, it has to do with who gets onto the Fencing Team, which practices AT Dupont. D'Argencourt just calls it his 'School of Fencing' because he's pretentious." Also, "He's the most prestigious fencing coach in Paris apparently but can't afford his own studio to train at, LOL." This explains how Adrien was taking fencing lessons before being allowed to attend classes at the school, as well as him choosing to enroll in Collège Françoise Dupont.
The Reveal that Bob Roth's plot to breakup Jagged and Anarka went undiscovered for years simply because the two never bothered properly talking about it explains why the record executive was so confident that he could get away with plagiarizing Kitty Section and tricking Marinette into thinking Jagged Stone wants to go in a new direction with his next album cover despite the obvious flaws in both schemes.
After defeating Robostus, Chat Noir and Marigold discuss the wish-granting powers of the Ladybug and Cat Miraculous and together conclude that "Hawkmoth can never win." While the immediate meaning—that they can't let the villain get his hands on the two Miraculous due to how his wish would rewrite reality—is clear, the two heroes could very well also be concluding that Hawkmoth would never be able to win in any meaningful sense even if he did get to make his wish because the Equivalent Exchange nature of how it gets granted means he'd essentially end up right back where he started.
How could Nathaniel draw a picture of himself as Caprikid despite not having seen his reflection or a photo for reference? Well, a Miraculous holder's appearance is based on what they imagine themself to look like, so he just drew that! Also, Nathaniel has more confidence at the idea of using a Miraculous than he did in canon, explaining the change in hairstyle so that more of his face can be seen.
In "Party Crasher", Lila immediately spots the lies that fooled everyone else, but how did Sabrina do the same? Easy: she's been dealing with Chloé's bullshit for years and fixing it for her, so she's used to that.
"Party Crasher" also keeps Chloé Out of Focus for most of the episode: she doesn't appear until a bonus panel fifteen pages into the twenty-page episode. Yet it takes Party Crasher pointing out Scarlet Lady's absence for anyone to notice or care, which makes perfect sense; by this point, everyone present is aware that all the Nominal Heroine is really needed for is casting the Miraculous Cure... and Party Crasher reaffirms that Hawkmoth wants the Ladybug earrings and Cat ring most of all, with all of the other Miraculous being frivolous extras. Of course the villains are more bothered by her absence!
On top of this, when she does show up, Chat Noir casually remarks to Viperion that it's more a matter of when she messes up their plans rather than if, offering another reason why the villains would be more bothered by her absence than anyone else: she works in their favor in more ways than one.
When Marigold instructs everyone to dance while fighting Party Crasher, Scarlet Lady winds up being hit despite being legitimately good at dancing... or rather, because of that: the whole point was that they were meant to move unpredictably, but Scarlet Lady missed the point and opted to show off her classical training, making her movements perfectly predictable.
It might first seem odd that Sabrina was able to get past all the hotel security and staff in "Party Crasher" on her way to Chloé's room since the two of them stopped being friends months ago. However, "Malediktator" reveals that Chloé is still convinced that Sabrina will eventually come crawling back after realizing she needs Chloé. Thus, Chloé likely told the staff to just let Sabrina up to her room if her former friend ever comes looking for her.
In "Frightningale", Alya, Juleka, Rose, Mylene, and Kagami, all of whom have exactly one battle each under each of their belts, put together an Indy Ploy that nearly successfully takes down the titular akuma who was giving the more experienced Marigold and Chat Noir trouble. Then Scarlet Lady arrives to try and claim victory in another attempt at Stealing the Credit, which gives Frightningale an opportunity to counter-attack and escape. The heroes, permanent or temporary, are very much aware that she's a massive liability in the field and likely only have her involved if the akuma has done enough damage to require the Ladybug Miraculous to repair things.
"Frightningale" also has Marigold and Chat Noir refer to Lila and Sabrina as each other's "bestie", a title you would think belongs to Alix due to her efforts to befriend both girls and role in the formation of the Girl Posse. However, Alix already had friends like Nathaniel from class, who is probably her best friend since they were going to spend his birthday together. Meanwhile, Sabrina didn't have friends beside Chloé, who treated her as a lackey more than anything, while Lila was a New Transfer Student and thus didn't have any history with the class. It makes sense that the two would likely gravitate towards each other more than anyone else.
Zoé's initial obliviousness to her sister being Hated by All actually makes a lot of sense for two reasons. The first is that Zoé has seen that Audrey, who has a similar attitude, is still a world-famous fashion designer and critic, meaning that, for all Zoe knows, it could be the same with Chloé, where, at the very least, people respect her in spite of her attitude. The second was that Chloé, who has been shown repeatedly to be unaware of how hated she is, no doubt fed her sister an altered version of her school life and told Zoé that she's popular and that "everyone loved her."
During "Queen Wasp", Zoé manages to avoid getting caught via wearing a trash can over her head. While this might seem stupid, the next page shows Queen Wasp refusing to chase the heroes when they jump into the river due to how gross it is. Zoé may have assumed her half-sister would refuse to touch her if she wore the trash can.
At the end of "Queen Wasp", Tikki realizes that, in spite of the rule in place about it, Marigold and Chat Noir know each other's secret identities. Yet it's implied she doesn't out them to Master Fu in spite of being a stickler for the rules. Why? Well, she just got her true holder back and said true holder lives in a bakery. Rules or otherwise, Tikki isn't about to give up that after barely any time.
In "Catalyst", a conversation between Gabriel and Nathalie reveals that he originally planned to akumatize Lila during Heroes' Day, exploiting her enduring grudge against Scarlet Lady. He had no way of knowing that the now-former heroine would have fallen from grace by that point.
More importantly, the existence of this plan underscores one of Gabriel's biggest Fatal Flaws: how he doesn't care for others beyond how he can use them. Lila has grown immensely over the course of the series, yet Gabriel is completely oblivious to everything positive about her, only focused on the parts he can potentially twist to his own selfish ends. This mirrors how he's completely ignorant of his own son's personal life, to the point that he doesn't seem to be aware of his Relationship Upgrade with Marinette or react to Nathalie's slip of the tongue. The only time he acknowledged Adrien as a person in his own right was when he briefly suspected he was Chat Noir.
This point is further driven home when Sabrina and Lila manage to avoid immediately being caught up in his mass akumatization event. Scarlet Moth promptly whines to Catalyst that it's unfair he's not getting "my Vanisher and Volpina"; in his eyes, they're just Unwitting Pawns to play with as he sees fit.
And the logical conclusion is his complete inability to see that his new Scarlet Akumas can be purified through friendship and love, in spite of the precedent of Marigold being broken out of Dark Cupid's control exactly that way, until he's already lost five Akumas (one to Sabrina and Lila, for extra irony) and he's about to lose Dark Cupid himself to his girlfriend.
Adrien is a lot more willing to break off his friendship with Chloé than in canon - by "Despair Bear", he doesn't even consider giving her a chance to fix things. In the show, having Ladybug for a partner helped reinforce his faith in the goodness of the people around him - including the chance that Chloé isn't all bad. Here, however, he has to put up with Scar, which drains his patience with Chloé's nonsense a lot faster since he's dealing with it on both sides of the mask.
Chloé starts suffering from the effects of the Peacock Miraculous far quicker than Nathalie did, beginning coughing up blood and passing out after only using it for a few hours when compared to Nathalie, who managed to avoid any really negative effects for a large amount of time. Chloé is much younger than Nathalie and thus, likely, has a weaker immune system than an adult.
