This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Willy Wonka and the Chocolate Factory
The Oompa-Loompas are just little people in the book and other versions, but in this film they're orange-skinned, green-haired little demons. Your child was just mutated by a freak accident? That's terrible. Say, this will cheer you up: Men with irradiated skin singing in monotone about how he or she deserved it and how terrible a parent you are!
Even if the Oompa-Loompas themselves don't scare you, the way that they're first mentioned to the audience is pretty deeply unsettling. Think about it: a giant factory has stayed closed for decades, and absolutely no one knows what goes on inside...but every so often, people see the silhouettes of some inhuman, dwarf-like creatures in the windows, with no way of knowing what they are or where they came from. "We daren't go a-hunting, for fear of little men," indeed...
The Wonkamoblile foam scene seems to take a concept of total wackiness, the wacky car, and distort it into disturbing madness. All the odd bit make a really jarring noise to young ears but it's probably because of the way Mrs. Teavee kept screaming.
The scene where Grandpa Joe and Charlie drink the Fizzy Lifting Drinks and Charlie almost gets axed by the fan.
The first thing people of a certain age will mention when the topic of "things that scared the crap out of you as a child when you didn't expect it" will be the "There's no earthly way of knowing/which direction we are going" sequence of Willy Wonka and the Chocolate Factory. No child alive saw it coming. It remains one of the ultimate examples of Nightmare Fuel to this day. It even made Bravo's 100 Scariest Movie Moments list (it was #74).
Wonka: There's no earthly way of knowing... which direction we are going... There's no knowing where we're rowing... or which way the river's flowing... Is it raining? Is it snowing? Is a hurricane a-blowing? ...Not a speck of light is showing, so the danger must be growing... By the fires of hell a-glowing...is the grisly reaper mowing? YES! The danger must be growing, for the rowers... keep on rowing... And they're certainly not showing... ANY SIGN THAT THEY ARE SLOWING! RRRAAAAAAAAAAAAAAAAAAAGGGHH!
How is the boat ride anything but? You're stuck in a tunnel where you get flashes of insects crawling out of people's bodies, spiders, and a man butchering animals, all while Gene Wilder appears to be going insane. It was even nightmare fuel for the other actors — they were told that they would be on a boat, and that Gene would do something, after which they had their lines. And that's all they were told! When everything goes shit crazy and people don't seem to be able to even speak right, it's because they CAN'T.
The "nice" thing is that this is the only song whose lyrics are actually taken from the book.
The look on the character's faces were genuine fright; the actors weren't told that Gene Wilder was going to start monologuing at that point, much less doing it in such a weird and creepy manner. That's how terrifying the scene was.
When Disney Channel ran the movie in the 80's and 90's, they kept this scene (though they cut the chicken decapitation part) and slapped a content warning in front of it, literally the only time this was done for anything that wasn't rated PG-13 or up.
Willy Wonka himself is pretty freaky at times — even — scratch that, ESPECIALLY — when portrayed by Gene Wilder. Aside from the aura of barely-veiled madness, the guy engineers some pretty disturbing fates for his ill-mannered guests. (Not that they didn't all deserve it, but it's still disturbing to watch.) His unconcerned attitude toward the horrible demises of his guests are certainly unnerving, along with his general Nightmare Fetishist behavior. Memorable quotes include:
Wonka is a mad genius who engineered a wonderland that's irresistible to kids, for the purposes of punishing the bad kids (and, very nearly, the protagonist) in quasi-magical but still pretty horrific ways. What does this sound like?
Augustus falling into the chocolate river and being sucked up into the clear tube, where he gets stuck. Drowning, suffocation, trapped in plain sight, screaming for help but unheard, seen but not helped...Abandoned to deadly fate...
Violet getting turned into a blueberry. Sure, it's funny when you're older, but when you're a kid the only thing you can think about is that she might explode, and how the Oompa Loompas are pushing her out of the room, singing a song and seemingly indifferent.
Wonka: It always goes wrong when it comes to the dessert. Always.
And even the way Wonka grabs Veruca's tongue in the snozzberry scene is quite unsettling.
Wonka: We are the music makers, and we are the dreamers of dreams.
It can still be unsettling to watch the infamous meltdown scene in Wonka's office, especially due to how uncharacteristically angry he gets. No wonder when they ran it on The Disney Channel, they warned parents that the movie contains scenes that might be too intense for young children. (The boat scene might have sealed the deal there.)
Grandpa Joe was afraid when the Wonkavator was going to hit glass roof. Charlie and Grandpa felt horrified when they would be cut to ribbons. Well, They survived from the broken glass as Wonka warned them in caution to hold on tight.