I used to be as a young child. I thought that if I put my foot on the first tread of the stairs, the front door would pull itself off its hinges and come after me to kill me. If I touched any part of my body, then I had to touch the top of my head with the same hand within a few seconds, or I thought the part I'd touched would become diseased. At one point I thought that shaking my head or saying no would ward off back luck, while nodding or saying yes would invite trouble. These were completely my own ideas, and I never shared them. They held their grip on me because I had no proof that they weren't correct. Mostly I grew out of them, though to this day I can still feel very slightly uneasy if I touch a body part that I'm worried about.
I absorbed some of the folklore superstitions for a while - Santa, ghosts, the number 13, that kind of thing. I even had a strange dislike of all odd numbers, and a liking for the numbers 2, 4, 8, and 16, though I didn't associate them with luck. The school's scripture lessons had me believing the usual Christian beliefs. I looked into religion later, in the hope if finding out, and gradually came to the conclusion that the supernatural was pure fiction. My earlier programming still lingers very slightly - if I declare out loud that there is no god, I get a slight feeling that I could have jinxed myself. I also feel reluctant to walk under ladders.
Probably the oddest (and strongest) "superstition" I have is a feeling that complacency is very unlucky. I daren't crow about a victory in case it somehow gets taken away from me. I feel that I have to discuss risk in a very serious way, never dismissing it as ridiculously unlikely. Maybe it's from the biblical "pride cometh before a fall" thing, or maybe it's from all these movies and sitcoms where they so often portray people getting shaken out of their complacency for comic effect.
But consciously, I don't believe there are any supernatural beings or forces whatsoever. I think it's difficult to be human and have no trace of irrationality.