I feel like we all have a “I saw it in middle of the night on PBS and it really screwed with my head” movie.
For me it was being maybe 13 and tuning in to the last half of Lord of the Flies at about 1am. Those kids abandoned on a lonely island, followed by the rescuer showing up, followed by credits, followed by the national anthem and test colour bars will forever be burned into my brain.
My parents had recorded something on our VCR and "to be safe" it recorded the first half hour or so of the following movie. The following movie was the original Alien. It stopped around the dinner scene before it gets really going. I must have watched that part of the movie ~20 times when I was around 10. I'd be constantly afraid about facehuggers hiding under the bed or behind the shower curtain when it was dark.
For me that was Beyond the Valley of the Dolls. Roger Ebert was not just a movie critic... And it inspired Austin Powers as well!
For me it was the first letterboxed thing I had ever seen on TV and I asked my dad what was wrong with the TV. Feels like it was on A&E? At least 30 years ago.