Miraculous Ladybug/Nightmare Fuel

The show in general

 * How akumatizations work: Hawkmoth essentially takes advantage of a person's darkest moments and brainwashes them to become his plaything. Not only does he akumatize anybody including small children and babies, but if someone wants to use their power for good or simply wants to back out (and it's not because they're petty), Hawkmoth will force to them do it (thankfully, it's shown people can successfully break akumazations in later seasons).
 * A lot of Marinette's stalkery antics can be this.
 * A lot of the faces she pulls regarding him can be this as well.

Stormy Weather

 * The first akuma we see in the show slips through into the elevator where Aurore is. Unlike most episodes, where they don't notice or fight it, Aurore actually (understandably) seems scared of it and tries to fight it off. But it doesn't work and she becomes Stormy Weather.
 * The scene where Ladybug and Chat Noir are frantically dodging vehicles, particularly a bus.
 * The ever growing icicles that threaten Alya and Manon.
 * The beginning of Hawkmoth's gross treatment of underage people. He very affectionately calls Aurore/Stormy Weather my weather girl. Ew!

The Bubbler

 * Like most 'get rid of the adults' scenarios in media, many viewers were quick to notice and worry about what happened to any babies and young kids after their parents/guardians were abducted by Nino.
 * Where were the bubbles taking the parents/guardians/unhappy kids? Oh nowhere, just outer space!

The Pharaoh

 * Jalil turns into the titular villain and by doing so, gets the abilities of the Egyptian gods including turning people into mummies (including kids), and tried to sacrifice Alya, who comes dangerously near to being hurled into total darkness.

Lady Wifi

 * Alya transforms into Lady Wifi and gains the ability to traverse Cyberspace (i.e. most phones), which means that if she wants you, she'll get you and you won't even know it until it's too late.
 * In one instance, we see Sabine and Tom watching the Lady Wifi broadcast scared and worried. Made worse when you remember, Marinette never came home from school, having transformed there.
 * If her hunch would about the source of Ladybug's power being their masks had been correct, it could be inferred Lady Wifi was probably intending on leaving Chat in the freezer to freeze to death, so she could take his miraculous once he had passed.

Timebreaker

 * Unintentional example: When Alix shows up to the race, everyone's happy to see her. The problem is some of the open smiles they have make their mouths too big in contrast to their teeth making it an eerie uncanny valley. Marinette and Alya specifically have this problem.
 * Anytime Timebreaker smiles, it's creepy and makes her look even scarier. One example is when Ladybug saves Ivan, but Timebreaker is able to pull Ladybug towards her and we see an amazing view of Timebreaker standing over Ladybug, smiling wickedly.
 * Timebreaker's main gimmick: she takes other people's energy to use to bring herself in time. When she touches them, they slowly fade away.
 * The french dub, in particular, makes her sound terrifying. She snarls and hisses like some sort of wild animal.
 * One of the first things she does is manipulate Rose into thinking she's sorry, thus tricking her into giving her her energy. Timebreaker even mocks her for it afterwards!
 * Just the fact, Timebreaker was one of the first akumas to almost win. She was close to taking Ladybug's earrings if Chat hadn't decided to sacrifice himself. Similarly, she could've gotten Chat's if she didn't choose to go back in time to try and get her watch first.