In the 80s there was a game on the Commodore 64 called 'Shamus' - if you stayed in a room too long a character called 'The Shadow' would appear from nowhere and rush at you; if he touched you he'd kill you. For many years I would apply this theory to certain rooms and would hurry to get out of them as quick as I could... mostly those rooms where I was alone such as the bathroom or the stairs.
Of course I don't believe it anymore, but I believed it for long enough to develop habit that I still have now I'm 27; I nearly always run up stairs...
A childhood belief about monsters

others believed things like this
- I believed that teachers lived in rooms above the school gym.— Kelly
- I used to think a female daddy-long-legs was called a "mummy-long-legs".Anon
- When I was little, I believed that if I didn't go to the bathroom quick enough, I would actually explode.— Coco S.
- I believed that the breed of dog called Boxer was called that because they could stand up and box.Anon
- I used to believe if you stayed in water too long your skin would get really wrinkled and stay that way.Anon
- When I was little I thought that if I stayed a long time in the water I would become a mermaid.— Salma