I used to believe that everyone who had the same first name must also have the same middle name. My younger brother is named Matthew Scott (we call him Scott) and I assumed that every Matthew I met was also Matthew Scott. (I have an uncommon name, and never met anyone else with it and that was only more proof somehow.)
A childhood belief about names

others believed things like this
- When I was little, my dad had a friend named Scott. I thought my dad's friend was the king of Scotland— Uhhh....
- I thought Scott Joplin and Janis Joplin were related. Two musicians with the same last name - it made sense!— Rita
- When my cousin was younger his name was Alexander and he thought his middle name was "ander"Anon
- My 18 year old brother thought the former vice president was named Al Gorbachov.— Jeff
- I thought Kaa's name from The Jungle Book was spelled "Car" and that he had a brother named Truck.Anon
- i used to believe that bras were bullet-proof vests for women.— emily