When I was young (though not as young as I'd like everybody reading this to believe), I believed that white dog poo came exclusively from poodles, and no other make of dog. I can't remember why. It turns out, perhaps unsurprisingly, that it wasn't true. It was related to the amount of chalk in dog food, which is now banned.
A childhood belief about cats and dogs

others believed things like this
- As a young child I thought that sheep and poodles were the same animal.— Jane
- I used to believe that toy poodles were actually normal poodles shrinked by a shrinking ray — nightmare
- I used to think that if you ate cat food you would die. I found out it wasn't true
— Bob#4 - I believed when young that horses with white faces were sick. My dad a great sense of humour— John Cavanagh
- When I was young, I said to my mom, "Mommy, make the sun go down!"Anon
- I used to believe that all my aunts were related to my mother and all my uncles were related to my father.— Mandy