This doesn't really fit into any of the bad habit categories, but I've got to confess.
I used to have a habit of picking the fluff out of my belly button whilst sitting watching TV.
My mum tried to deter me from this practise by telling me that if my constant fidgiting made my belly button loose, my bum would fall off.
All this succeeded in doing was keeping me rooted to the couch in fear that if I stood up, I may well leave my bum behind.
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