I used to believe that all autobiographies ended in a string of gibberish because I only ever read biographies of dead people so I assumed that an account of the subject's death was a necessary part of a biography and there was no way someone writing an autobiography would be able to get out the full account of their own death before collapsing at their keyboard.
A childhood belief about history

others believed things like this
- I used to be really confused about the whereabouts of God.
On account of: "Our Father who Aren't in Heaven".— Tania Bradshaw - I used to believe that a street sign that read "Dead End" meant that there were dead people buried there.Anon
- Before I knew about death, I used to think people turned back into babies again after they got old. Anon
- I used to believe that the sky was full of dead people and I would see them if I went high enough— Maria
- I used to believe that death come to visit you. — Lesley
- I used to believe that if you bled to death, you'd get flat!— Lisa G