When I first heard the word "suburb" I thought it meant the bad parts of town, since I knew "sub-" meant "under" or "less than" (i.e "sub-zero") so I thought it meant the part of the city that was less good than the rest.
A childhood belief about excursions

others believed things like this
- For years I heard the word "pedophile" and thought it meant a pedicurist's tool.— Lauren
- I used to believe that "nuptial" was a naughty word. I thought it meant nipple. XD— Sara Hagen
- I thought that a person's motorskills meant how good they were at driving!Anon
- I thought when someone was described as being "mild-mannered" it meant they didn't have very good manners— Samilton Duerheiser
- When I used to see a sign saying "Littering Fine" I thought they meant if you littered REAL GOOD!— Lou V.
- i used to believe that sex meant love and that having sex meant the two people would live happily ever after.— amy