I was afraid of picking my nose, because I believed (and vaguely recall being told) that snot was fly poop. Because, (of course!) when flies shat in the air or in our vicinity, it somehow got ingested through our nostrils. :S
A childhood belief about picking my nose

others believed things like this
- when I was little I was afraid of flies until mom told me they were baby birds!— Lisa
- I used to think bogies were made of dead flies which had flown up my nose.— Kate
- I used to think, that if you're picking in your nose too much, you push your eye out.— Joanna
- When I was younger, I used to think that picking your nose was against the law.Anon
- I used to think that if you didn't blow your nose and just sniffed the snot up, you head would blow up.Anon
- Picking your nose and eating it. Hey! Tastes like chicken and only half the calories— Henry the Seventh