This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Hey Arnold!
Well, it wasn't like Cousin Arnie was creepy enough already.
"Part-time Friends". Arnold and Gerald work together at a flower shop with Gerald being the boss since the owner injured her leg and appointed him as the temporary manager. After Arnold gets into a fight with Gerald and says that he doesn't want to be friends with him, Arnold's Grandpa tells him about a similar incident that happened to Grandpa himself when he was Arnold's age. Arnold then begins to have a nightmare dream where he and Gerald are old, still former friends, and can't remember what they were fighting about. The dream takes a turn for the disturbing when it cuts to the old, withered, zombie corpse of Arnold's Grandpa where he says "Well, what did I tell ya, Shortman?" while some ominous music plays. He then proceeds to laugh in a really, creepy way, only for his jaw to crack and fall off his skull.
Curly's character while can come off more as Affably Evil at times really goes off the deep end at times and has cemented himself in the eyes of many viewers as a seriously disturbed, troubled, and possibly insane individual. Remember the episode where he snapped simply because Mr. Simmons made an honest mistake in not picking him as the ball monitor and he locked himself in Principal Wartz's office while throwing balls at people? Hmm, doesn't that instance come off as a little too close to a school shooter?
There was also the episode "Principal Simmons", where Mr. Simmons becomes principal of the school but fails at it because he refuses to punish students and every student in the school save Arnold disregards authority and totally trashes the school. Mr Simmons enters his office to find Curly strapped in a fire extinguisher which he promptly ignites and goes FLYING OUT THE WINDOW AS IF IT WERE A JET PACK!!! How the hell did he survive a flight out a five story window at that velocity without any injury?!
Curly casually mentions in one episode to Rhonda that he hadn't changed his underwear in five weeks. He promptly then asks her for a kiss.....yeah, nothing strange about that at all.
And of course in the episode where everyone, but Arnold is invited to Rhonda's "cool kids" party and they decide what to do to get even with Rhonda. Curly's response? Paint themselves with tiger stripes and go free all the animals at the zoo. His peers....react to this as any sane person would. Funny thing is at the end of the episode, Curly is seen painted in tiger stripes and riding a giraffe with a legion of animals on streets....
And earlier in the episode when Eugene said something in the lines of nobody biting the heads off chickens, except for Curly. And Curly replied it was the one time. Curly bit off a chicken's head!
Despite much of the sympathy in the episode Curly's Girl being directed at Curly himself, he still blackmailed Rhonda into being his girlfriend for a week. Add that to the disturbing infatuation he has about her and one has to wonder what it would have like had they been just a little older.
Considering that leaking nudes, whether it be carelessly sharing them after gaining someone's trust without really knowing the consequences or purposeful sextortion, is a hot topic nowadays, this will probably hit a nerve with some people who have been blackmailed with "pictures" in order to do more for someone (and in Curly's case, it's definitely a metaphor for sextortion).
There was one episode where Arnold and the gang are telling scary stories. Arnold's tale is called "The Headless Cabbie" which involves a cabbie driver picking up a woman who wants to go to the park because she's lost her little dog (she even gives the cabbie a scarf since it's late at night and he won't be cold). As the cabbie drives through the park, he hears barking, and the lady says that's her dog. The cabbie keeps driving, the barking continues, he doesn't see the little dog, but the lady keeps imploring him to go faster and faster, sounding and becoming more demonic. The story reaches its climax when the scarf gets caught on a tree branch, and since the cabbie is going so fast, the scarf winds tighter and tighter... until it finally cuts his head clean off. Not surprisingly, Harold is traumatized by the story. The ending of that episode is pretty scary through implications. The crew decides to take a shortcut to the ice cream parlor that same night through the park, and all the while Harold worries that they'll see the ghost of the decapitated cabbie. As they follow the route the cabbie took, more and more elements of the story starts to show up. They see a hook-handed man past a bridge, find a little terrier dog, and - in the big scare climax - hear a cabbie riding up to them, with a woman cackling evilly in the carriage. The kids then learn that everything had a rational explanation. The hook-handed man was actually seen in shadow holding a watch display and the cabbie driver was just Ernie, taking a job for extra money, and the creepy woman laughing was just Mr. Hyunh... laughing creepily for the heck of it. As they leave though, a woman approaches Ernie explaining that her dog has gone missing and she wants to know if he can give her a ride to find it. Ernie agrees and also accepts the scarf she offers him...
The town and the school flooding in "The Flood". Worse still, all the kids are trapped in the school.
Arnie and his horrific post-nasal drip. And that parallel of Arnold's pig was horrific... everything about the Cousin Arnie episode was absolutely horrific, actually.
Arnie's town, essentially a Bizarro Universe counterpart to Arnold's town, has an immensely eerie tone to it...and then we have the very end where Arnie thinks Arnold is trying to steal his girlfriend and absolutely flips.
Nobody can forget the Ghost Bride episode. It was insanely creepy and scary, and it actually ended with the implication that a secondary character was killed by the ghostnote Which gets worse when you realize that Curly doesn't appear again after this.. Wow, how this episode was left in, one can wonder.
Plus, Arnold reassured everybody else that Curly would be just fine, despite him and the gang leaving him locked in a crypt after tying him up.
Also, the whole Ghost Bride's origin story. Her fiancee abandoned her on their wedding day, and he married her sister; this made the abandoned bride to literally go axe crazy and murder them both right in the middle of their honeymoon, and next morning, when the cops arrived, she was still there, throwing rice and humming the Wedding March next to the murdered couple, before committing suicide by jumping out of the window. All this was shown as graphically as censorship allowed it in a kids' network.
While the Wheezin' Ed episode wasn't that creepy, it ends with an asthmatic disturbing breath and laugh that ended everything as ambiguous as every "supernatural" episode.
It's the animation of Gerald's story that is creepy, particularly the scene of the shadowy figure of Ed about to choke a man. The horrific wet *crunching* noise sounds disturbingly realistic, too.
The episode "Das Subway" could also count. Not only are everyone stuck in a subway inside the tunnel during the night, there is also the sight of black rats with red eyes crawling outside of the train.
In the episode, "Sid The Vampire Slayer", Sid accuses Stinky of being a vampire after being rattled by a vampire movie. Arnold insists Stinky couldn't possibly be a vampire and goes to Stinky's house with Sid to prove it once and for all. Sid is more than willing to 'eliminate' Stinky, but after seeing his reflection in a mirror, decides he couldn't be a vampire after all. However, after Arnold and Sid leave, Stinky is seen talking to a bat about how two guys came by accusing him of being a vampire. Immediately after this, Stinky is seen GROWING HUGE FANGS WITH NO CUTAWAY, and laughing maniacally. This never comes up in the show again, and happens in the final season.
"Phoebe Cheats". The Emily Dickinson statue is creepy all around, especially when it taunts Phoebe into admitting she plagiarized her contest-winning poem, "CHEEEATERRRR...CHEEEATERRRRR......Cheater!"
this is somewhat downplayed when Phoebe takes the statue out and it asks if it'll need a sweater
"Four-Eyed Jack" ended without the usual ambiguity that the other supernatural episodes ended with. That is to say, we find out the ghost is DEFINITELY real, and though he doesn't appear to be too malicious beyond scaring Gerald for kicks, he still looks extremely creepy.◊
The episode where Helga believes she is dying from "Monkeynucleosis". While Helga's situation is Played for Laughs, for the most part, the same can't be said for Helga's Nightmare Sequence about turning into a monkey midway through the episode.
The scene in the nightmare when Helga turned into a monkey, the creepy music the organ grinder was playing while she was dancing was a bit unsettling.
"Longest Monday" can hit a little too close to home for those who have grown up experiencing the rituals of hazing, especially of the "bullying for the sake of tradition" variety commonly directed towards freshmen in high schools.
And who could forget the Snee-Oosh logo and theme song at the end of the Nick split-screen credits?
In the spelling bee episode, one kid gets caught cheating. He is hoisted off the stage by a huge bodyguard in a surprisingly intense scene.
The "Pigeon Man" story Gerald tells in one episode; the sinister "alien music" is spine-tingling.
The "Stoop Kid" story is scary too, for similar reasons.
The 'Haunted Train" episode. Grandpa tells the kids a story about a train engineer that suddenly went insane and drove his train off the rails, killing everyone on board. On the anniversary of the event, The ghost comes back on the train, hypnotizes anyone who happens to be at the station into boarding, and drives them INTO HELL WHERE SATAN HIMSELF BOARDS TO DRAG THEM OFF. Later, the kids go to the station to see if this is true, they get on a train (which later turns out to be a train for steel mill relief workers) and Brainy shows up OUT OF NOWHERE after they are on a moving train and he says he doesn't know what he's doing there. Later, after they decided the ghost train was just a myth, Brainy is seen sitting on a fence watching another train passing by, and the ghost engineer (whose appearance is terrifying by itself, and bears a passing resemblance to Leatherface) is shown sitting in front of the locomotive, singing the song about the haunted train Gerald sang earlier on. Sweet dreams.
The monitor lizard eating the parrot at the end of "Helga's Parrot" can be a little unsettling to younger viewers. Hell, even some older viewers find it unsettling.
There is a brief one in "Helga and the Nanny," in Helga's guilt-laden dream where she meets the recently fired nanny in the park. Towards the end, Inge is reduced to fighting for scraps with pigeons because she can't find a job.
Helga's crush on Arnold falls into this at times like when we see the lengths she is willing to go to in order to keep people away from moving in on Arnold and how she pulls out all the stops to keep her crush a secret and when she threatened to strangle Lila if she ever revealed to anyone she has a crush on Arnold.
Her stalker shrines of Arnold and way of obsessing over him are extremely creepy. There's even one she made from his chewed bubblegum. And let's not forget the one from The Movie that is mostly made out of a taxidermy bear.
This is more of a Nausia Fuel than Nightmare Fuel, but since her dreamy sighs sound a lot like The Modest Orgasm and her poem in "Helga's Parrot" could pretty much be titled "Arnold Makes Me Horny", it makes you wonder just how innocent Helga's intentions are, especially disturbing since she's only nine.
In the episode "Chocolate boy" there was a scene when Arnold showed Chocolate Boy what would happen if he doesn't give up his addiction to chocolate, it was a slide show of people vomiting, having rashes, anyway it was really disturbing. Not that the Boy is really fazed by it.
In the episode "Sid and Germs" Sid's nightmare can really be disturbing for people who have a fear of bacteria, and the nasty bugs crawling all over Sid's bedroom was a bit creepy and disturbing also.
In the episode "Mugged" Arnold was robbed by a local hoodlum in the opening of the episode. There was nothing cartoony about the scene, it was an terrifyingly accurate portrayal of a person being put in that situation.