My mother told me (when I was wee) that if I stuck my finger in the pudding to taste it, it wouldn't get thick, but instead would be all runny and yucky so she'd know I'd touched it.
I was 28 before I finally asked anyone else if this was true (via a post on usenet). It isn't.
Now I stick my fingers in the pudding on general principle every time I make it.
A childhood belief about general

others believed things like this
- For the longest time I thought that tapioca pudding was really fish eggs in pudding.— Liz
- I used to believe that the rice in Tapioca pudding was fish eyes. To this day I cannot eat Tapioca pudding.— Stephanie
- My father told me tapioca was frog eggs in pudding. I still can't eat it!Anon
- i used to believe, and was told, that there was fish eyes/eges in tapioca pudding. I have never aten it since.Anon
- My Nan used to say that if you ate rice pudding every day you would live till 100.— Jayne
- I used to believe that tapioca pudding had fish eyes in it! EEEWWWWW!— Chris