When I was 6 I heard the song "Who can it be now?". I used to think the words were "Your getting bigger now!" instead of "Who can it be now?". My family burst into laughter when I sang it wrong.
A childhood belief about misheard lyrics

others believed things like this
- i thought I got one year older when my family sang the birthday song.— I am the ghost
- I was about 6 and thought my family invented the fart because I never heard anyone outside my family do it.— Bob
- for my little sis -
Paul McCarty song "Band on the Run" but when she sang it it was "Man has the Runs"Anon - I used to think that the actors on tv shows sang the show's theme song.Anon
- I used to think that in the song"Family Affair, they were saying "It's a family of bears!"— Wayne Huffman
- Until I was about 15, I beleived my Father was the person who sang the "Cheers" TV Show Theme Song.— Dustin