I watched The Evil Dead and Nightmare On Elm Street as a double at my friends house when we had a sleepover around age 11 or 12 his mum rented them for us
I wouldn't walk under trees for a while and the scene in Elm Street when Tina appears in a body bag during Nancy's dream in class and the centipede crawls out of her mouth... eugh - scared the crap out of me!!
I also remember being more scared of The Exorcist before I'd seen it, and feeling dread during watching it but by halfway through I realised it wasn't actually scary.