I also was told by my older brother that if you unscrewed your belly button your butt would fall. I remember thinking that was stupid, but as my brother had told me that, it had to be true!
I'm from Spain, so I guess it's a worldwide belief...
A childhood belief about belly buttons

you weren't the only one
97 of you believed this
- I used to believe that if someone unscrewed my belly button, my butt would fall off— David
- i was told that if you fiddled with your belly-button too much, my bum would fall off.Anon
- My Dad told me that if the knot in your belly button came undone that your bum would fall off! — Sue, Staffordshire, England
- when i was young i was told that if i played with or turned my belly button that my bum would fall off — lee hull
- That if you fiddled with your bellybutton it would unscrew your bum and it would fall off— Sundaeg1rl
- I believed that if you pushed your belly button, your bum would fall off.— Wendy