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.
Hmm I guess I've only seen the edited-for-TV version of that movie, they don't show the body. Which reminds me, that's the original name of that story, based on a Stephen King novel called The Body.