When I sang the American national anthem as a child, I thought that the line, "...by the dawn's early light" was actually "...by the 'dawnserly' light", and 'dawnserly' was a word that meant something aking to 'beautiful'. My parents thought it was cute, when I used 'dawnserly' in a sentence as a substitute for beautiful, and laughingly corrected me. I was a touch ashamed, though, and disappointed that such a word didn't actually exist.
A childhood belief about misheard lyrics

others believed things like this
- 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 believe that "nuptial" was a naughty word. I thought it meant nipple. XD— Sara Hagen
- For years I heard the word "pedophile" and thought it meant a pedicurist's tool.— Lauren
- I used to believe that the yellow light in traffic-lights meant Go Faster!— M. L. E.
- my husband believed, until recently, that the word "several" meant seven of something. — sheryl
- I used to think the word fever was just another word for giraff— Marshall m