I used to believe that to add, for example, seven and three, you'd count "seven, eight, nine". Obviously, adding one to any number didn't change it.
A childhood belief about counting

others believed things like this
- I thought that Count Dracula was the count from Sesame StreetAnon
- when i was little i believed that your social security number was the number you were born on the earth — christine
- I used to believe that adding salt to food will make it cooler.— Ale Nijamkin
- When I was a child, I used to believe that adding sugar would lower the over-salty taste of a meal.— Francois
- that people in shops gave you 'change' if you didn't quite have enough money to pay.— suzy
- adding too much sugar to foods would give you "sugar worms"...
Anon