I used to believe Tim Burton was the son of Richard Burton. And not only as a child, I was about 20 when I found out they're not related at all.
A childhood belief about films

others believed things like this
- I used to believe as a child that a Communist and a Columnist were closely related, even one and the same.— Elaine M. Downey
- I used to believe that all my aunts were related to my mother and all my uncles were related to my father.— Mandy
- I used to think "from which it stands" was "for Richard Stans".
— cobra - I always thought that Little Richard was Cliff Richards younger brother— adam
- Until recently, my friend believed that Richard Starkey was the fifth Beatle.— Spook
- I thought the Virgin Islands were owned by Richard Branson until I was 24. — LJ