We'll it doesn't haunt me but looking back it was RoboCop. That movie is a bit much for a 6 year old to watch.

Growing up in the 80s meant that pretty much any kids movie was going to be traumatizing. Gremlins: horrifying. Neverending Story: emotional damage. The Land Before Time: can't think of dinosaurs without tearing up. It's like the whole movie industry was explicitly devoted to fucking us up.

Watership Down

My parents thought it was a nice cartoon about rabbits I guess. Weirdly, My nightmares where mostly about the intro with the special art style, mostly…

Akira. My father rented it for my brother and me because "animated movie is for kids". I was 4, and my brother was 3.

Event Horizon.

I know it gets a bad rap, but it is a cult classic in my book. The most perfect symmetry of science fiction and body horror since Alien(s). Add on top of that a fantastic cast, a mildly campy vibe, and it somehow manages to hold up well even today in my opinion. Even though it scared the fuck out of me as a kid, I have a weird nostalgia for it now.

Oh oh oh I know this one!

Glory! The civil war film! There's a scene where a union soldier takes a cannon ball to the head and it explodes in a gory mess. It was during a tour to Gettysburg, and I threw up on the bus after seeing it. Then they brilliantly played the Mel Gibson Patriot movie where a revolutionary also takes a cannon ball to the head, only this time it removes the head in slow motion and more detaches it than blows it to head smoothie

American History X. I wasn't ready for the curb scene.

When I was about 5-6 I had fever and couldn’t sleep. I lived in a apartment complex and my mom had the neighbour from the next apartment over for coffee so I was sitting in the neighbours apartment while they had the doors open into the hall. Well, there I was, sitting alone in the dark, watching some sappy teens have a heart to heart while suddenly the earth opens under one of them and it gets brutally eaten the fuck alive while the other one screams in panic and tries to rescue it. Had some unforgettable nightmares that night.

Fucking Tremors man.

The movie that actually fucked me up for a bit as a kid was some black and white movie about spiders that took over a small town. I don't remember a single thing about the movie, other than crates/the town absolutely covered in webs and people getting wrapped up like bugs.

Stand By Me - The scene with the leaches. As a kid in a small country town with nothing to do on weekends but run around and swim in the local creek, I was so scared to ever do that again.

Both of mine I later learned are comedy/satire:

Starship Troopers and Mars Attacks.

The sheer gore from Starship Troopers made me ill.

The Martian design was freaky and I wasn't a fan of the instant death lasers. It had me thinking aliens could come down one day and we'd have no chance against them.

The ring

it.

I still dislike clowns to this day.

My cousin and I used to spend the night at my grandma's house fairly regularly. Between my grandpa and my two uncles who lived there, the house had its fair share of old blank VHS tapes containing recordings of various horror films among comedy classics like Revenge of the Nerds. As far as horror goes, Return of the Living Dead scared the absolute shit out of us at age 8, as did Tremors and Gargoyles (1972).

And since you're no doubt wondering, I don't remember coming across any porn on those old VHS tapes, but my uncles did keep a few magazines stashed away in their closets that my cousin always knew exactly where to find.

EDIT: God damn, this really opened up a well of (positive) memories over an entire family that has since deceased. Cancer and poor health eventually took every last person in that house. Doesn't help that nearly everyone smoked and drinked to the day they died. They were all such good people, though. Rest in peace.

The Shining.

I remember asking dad "what's wrong with that man (Jack Nicholson)?" and he told me that he had spiders in his brain. That was also really weird.

Bambi (seriously)

the exorcist. At age 8

My Catholic church going up bringing has made possession into a genuine fair even though I'm atheist now.

Killer Clowns from Outer Space. That was one creepy fucking movie.

My friend and I went snooping and found her parents' hard-core sex tape. Should not have watched that at 10 years old.

Got a good quick lesson in anatomy, though.

I already mentioned one movie in a reply (Arachnophobia), but another that really sticks out, and which I watched at an even younger age is The NeverEnding Story.

I remember being around 5 or 6 at a friends' house, parents just left all the toddlers in the playroom in front of the movie and had their social gathering, meanwhile I'm terrified and hysterically crying my eyes out (I'm sure at least a couple of the other kids were too, but I can't remember)..
Artax in the swamp destroyed me completely, and the Darkness and the Sphynx statues, and even Morla scared the living shit out of me (yes, they left us there to watch the entire movie).

I still can't bare to watch the swamp scene.

Saving this whole thread for movie recommendations.

Twister. Living and visiting the Midwest USA surely didn't help. I used to get extremely anxious when it would get even mildly windy and still have a bit of a panic attack when a tornado warning/watch go off.

Event horizon fucked me up for a while

Not the scariest movie but Pitch Black. I was 7 and definitely didn't help my fear of darkness very much. Pretty neat movie as an adult but definitely get flashbacks when I see it. Also the movie Signs. Was scared of it as a kid but as an adult I find that movie absolutely hilarious.

But the worst is not a movie but a video game. I watched my brother play Resident Evil 2 when I was around the same age. I was absolutely terrified of zombies after that. Because of a few specific scenes in the game I refused to have my bed close any windows. Even at friends houses I'd rather sleep on the floor if the couch was too close to a window. That lasted until I was like 13. As an adult I still can't bring myself to play that game. I love Resident Evil and horror games in general, but as soon as I hear the music for the main lobby of the police station in Resident Evil 2 I get so terrified I have to turn the game off. Maybe I'll be able to play the remake...

Jaws when I was six. Even swimming pools make me uncomfortable.

Not a movie but The X-Files series.
When my little sister and me were at our dads for the week we used to take our covers and pillows and lay down between the TV and the sofa, which our dad slept on. He had the TV on basically 24/7 so we'd watch something together and he'd fall asleep and then me and my sister would move to our beds after a while but often falling asleep ourselves right there on the floor. We had been doing this for years and then they started airing The X-Files late at night on the channel we mostly had on. I almost always fell asleep last, so I ended up being awake for a couple of episodes and they really traumatised me. I remember being the only one awake and being so scared I didn't dare to even move my head or even breathe fully. I did tell my dad about it but he'd always fall asleep pretty early and I'd forget to change the channel.

Years later both my sister and me had gotten too tall to fit laying down between the sofa and the TV so we had stopped that tradition but my dad still always fell asleep with the TV on. They started doing reruns of the series and that damn intro music was so scary for me that I would have a battle with myself of just riding it out or getting up and walking down the dark hallway to change the channel. Both options were bad in their own ways. I still get shivers down my spine from the theme music.

Blow Out at about age 8. I was worried for ages that someone would strangle me and mutilate me in a public toilet.

Watership Down was worse though. Why anyone thought that was okay to show to children, I will never know.

Invasion of the Bodysnatchers. I was looking under my bed for pods for weeks!

It doesn't really haunt me, but when I was a kid I was up early in the morning and had nothing to do so I turned the TV on. And a black and white movie was showing. And I knew that those are funny. Like Charlie Chaplin or Laurel and Hardy. So I laughed my ass off as Miss Marple was horrified watching a woman being choked to death in the next train over.

Four Rooms. I was 14 but very sheltered and that shit was nuts.

A Nightmare on Elm Street when I was about 8 at a sleepover birthday party. All of us had nightmares after that and he's still my go to boogeyman.

Interview with the Vampire. I was waaaay too young to be watching that, and the scene where the light comes out and burns that one to a crisp scared the hell out of me. I remember having trouble falling asleep for a couple nights after that.

Not a movie but when I was 9, my grade 4 teacher decided to put on one of those ghost shows about a haunted Scottish castle and the story went that you could hear the bumping of a murdered body being dragged up the stairs in that castle. I told my dad and he spoke to the teacher and she told the whole class it was my fault we couldn't watch tv shows anymore so I got doubly traumatized. But fuck that, I had to run up the stairs in my house for literal years after that fucking show because I was afraid I would hear the murdered body.

Man I hate that fucking woman.

Event horizon, it still terrifies me to this day

Anaconda. It's not that it was a particularly impressive movie but for some reason it intensified my ophidiophobia.

It doesn't haunt me, but Full Metal Jacket. My dad rented it for family movie night when I was 10 or 11, and needless to say my mom is STILL mad at him for watching that with me in the room for the first hour. Worst part is my dad didn't know about it, he only knew R Lee Ermy from a show he did on the history channel called Mail Call that he watched with my sister growing up. So he never expected Ermy to shout that stuff.

Event Horizon. I couldn't sleep for days as an 11 year old! Love it now, but man, way too scary.