The song "My Country 'Tis of Thee" contains a phrase "Of thee I sing." I thought, when I first heard it (and sang it, along with however many other 5-year-olds) I thought that "My Country Tizzaly" contained the phrase "Of V.I.C," and I had no idea what V.I.C. stood for.
A childhood belief about misheard lyrics

others believed things like this
- Until about age 9, I thought there was a country called "Operaland" where everyone sang instead of talking.Anon
- I used to think that birds sang in different languages depending on the country they were in!— Sarah
- i thought that the phrase "jet lag" was actually "jet leg" until i was 17...— truffles
- I used to think Hungary and Turkey were the same country... I have no idea where that came from...— Skylla
- I thought the phrase "At one's disposal" meant you should dispose of the person or thing.— Geirskogul
- i thought I got one year older when my family sang the birthday song.— I am the ghost