This is a weird one, but I used to believe that when a couple got married, they got new phone numbers (given to them by angels, of course) with numbers that match.
A childhood belief about marriage

others believed things like this
- I used to believe that numbers had gender, that the odd numbers were boys and even numbers were girls. — Evelyn
- Numbers went up to 100 then restarted at zero.Anon
- The highest of all numbers was "a finnety."— Becky
- i used to belive that 1 to 10 were the only numbers and all others were made up.....— Eric
- I used to believe there was a limit on the numbers you could count — Carmen
- I used to be terrified of the numbers 18 and 19. I think I thought that when you turned 18 or 19, you die.— Emmy