Believe it or not, I used to believe the word gullible was fake and went off telling everybody it wasn't in the dictionary. It took me years to find out that the saying "gullible isn't in the dictionary" was only a joke, and the people I told it to thought I was just saying that joke rather than really meaning it.
A childhood belief about speaking

others believed things like this
- I thought when Grandma's hens were cackling, they were giggling about some private joke in the henhouse. — Not a farm kid
- i used to spell eight "ate" cuz of the stuped: why was six afraid of seven, cuz seven ate(8) ten joke...— 8
- In first grade, I thought that if I've never heard of a word before, then it wasn't a word.— Destiny
- I used to get mad when people used the word "native" because I thought they were making fun of my name!— Nate
- For the longest time I didn't know that Spinal Tap wasn't a real group. No wonder I couldn't find their music.— Louis
- For years I heard the word "pedophile" and thought it meant a pedicurist's tool.— Lauren