When I was little, I was confused with the decimal system. It's easier to show as an example how I counted: "..., twenty-eight, twenty-nine, twenty-ten, twenty-eleven, twenty-twelve", and so on. When the count reached "twenty-nineteen", the next number (twenty-twenty) just didn't seem to make sense. It took me over a year to realize that the number after 29 is 30.
A childhood belief about counting

others believed things like this
- I used to believe that twenty was the largest number in the universe.— Bernardo Fernandez
I used to believe that some insects had twenty lives.LOLAnon- Just this moment I learned that earwigs do not go into your ears, i'm twenty-four.— Sheila
- I used to believe there were numbers called "onety" and 'twoty." I thought "twoty-nine" came before twenty.— CC
- I used to think that any number with a decimal point in it was HUGE.— sm
- when i was little i believed that your social security number was the number you were born on the earth — christine