Mine was The Blob. I saw just a couple moments of the movie as a kid when my mom left me in the house to run down to the store. The TV was on something innocuous, but of course I changed the channel when she left.
The scene I came into was when the Blob was rolling around picking up people and things, and engulfed the dog. It was terrifying! Then mom came back in and I hadn’t heard her open the door, so I was even more scared by her suddenly appearing in the room. Was sent to bed, where I ruminated on that scene, and assumed thereafter that it was a super scary movie.
YEARS later I watched The Blob and laughed my ass off; it’s awesome classic cheese. But I can still recall the terror I felt from that illicit sliver of viewing at 4 years old.