I used to believe that each U.S. state's local phone company was named after the state the company was located. (This was in the late 1980s, after AT&T was split up.) I believed this because I knew that Indiana's phone company was "Indiana Bell" and Illinois had "Illinois Bell," so naturally I thought there was a "Florida Bell," a "New York Bell," and a "California Bell."
A childhood belief about telephones

others believed things like this
- I used to believe that Alexander Graham Bell was not just one person. I thought that they were three people!— J.D.
- I thought that the insurance company Aflac was owned by Ben AffleckAnon
- I used to believe that California was my house, not the state I lived in.— Katie
- I always thought that my relatives were really tiny, and inside my phone when I talked to them on the phone.Anon
- When I was little, I used to believe that Illinois (where I lived) was a countryAnon
- I used to think that a company had boats all over the world making waves, and airplanes making cloudsAnon