I've got a funny one, but it's not something that I myself believe, but my ex-stepfather....until he was almost 30 years old....
His mother used to tell him not to play or poke with his belly button, because it would rupture and his intestines and such would spill out....HAHAHAHAHA it was my mom, who finally convinced him that it was not true. But for a long time, he still believed it. My mom would poke at his belly button and he would actually get scared, and start whining. We used to laugh at him about that so much.
As far as me, I can't think of anything. I was a weird kid, though, I never even believed in Santa, or the tooth fairy or anything like that.