I guess this can be put among the make-believe-s, but couldn't actually find a proper place for it.
You know that elderly believe in so much funny stuff and they get terribly worried when you wouldn't follow their "valuable" advice.
When I was at late kindergarten age I was often told by my grany that I shouldn't sing while having a meal - the cause of that would be my getting married to a man who often gets drunk... That was making me remain silent till the end of our meal, because I didn't want to feel guilty if my husband were an alcoholic.
What a way to shut my mouth up!
A childhood belief about songs

others believed things like this
- I usd to believe that at restaurants that had a BYO sign out the front, you had to "bring your own" meal.— Fiona
- When I was a child, I used to believe that adding sugar would lower the over-salty taste of a meal.— Francois
- I used to tape my mouth shut at night in case a spider would come along and make a web in my belly.Anon
- i used to believe that a sign in a cafe bone-fide meal meant meat with a bone in it— roy
- I used to believe that if you told a lie, Dragonflies will come and sew your mouth shut. For real.— Casey
- My husband used to believe that kindergarten was called, "kiddie garden." — wife of mispronouncing guy