Worms and Uncooked pastry.
My Grandmother told me that if I ate uncooked pastry it would give me worms. All she had to say was "Don't eat the dough." When I was 7 or 8; during a class cooking day at school, I touted my belief to a number of my peers (about 7 or 8 years old). The teacher overheard and dismissed my belief. I felt really silly and embarrassed.
Kids are programmed to believe the things adults say.
A childhood belief about dangerous food

others believed things like this
- My family told me that if I ate uncooked oatmeal that it would swell up and my stomach would explode.— david
- I used to believe that crocodiles were green because they were uncooked— The Ice Cream Sandwich Monster
- I always thought that the gummy worms were real worms that had been mummified— The Ice Cream Sandwich Monster
- I used to believe that eating raw potatoes would give you worms, like dogs get.— A Wild Cowgirl
- I usesd to believe that eating orange peel would give you worms. so used to eat it anyway out of spite.— troy
- When I was a kid, I used to believe that cadbury cream eggs real uncooked eggs in the middle Anon