This may not be a belief, and yet at the same time almost a third of this site seems to be devoted to bad parenting, so this fits well.
To this day, I have a tendency to pick my nose and eat the boogers. I've learnt to do it in secret now, because my parents were complete jerks about it. Yes, my own parents would always tease me about it.
They would ask me questions like "are you digging for gold in there, or diamonds?" They also said other things, but I forget what they were. I never actually believed there were precious minerals in my nose, but they did hurt my feelings.
A childhood belief about picking my nose

others believed things like this
- One day i told my mom that i wouldnt pick my nose until the boogers made it look like i was a walrus— Lotta Thang
- I used to belive that there was gold in my nose because pepole would always say that i was "diggin' for gold"!— addicted nose picker
- I used to think there were little booger factories in your nose that made boogers.— Melith
- My brother used to believe that dried out boogers were actually "paving" the inside of your nose. — sm
- Boogers came from magical booger fairies that mined them over night and put them in your nose in the morning.— Vivian
- if you pick your nose, there will be a tune from itAnon