When I was a kid, my "cool" older cousins always used to listen to AC/DC and there's a song on Back In Black
(Givin' The Dog A Bone) that I used to believe had something to do with the smurfs. When they sang the chorus, I thought they were singing "Give it to Gargamo! Yeah! Give it to Gargamo!" I thought Gargamo must have been Gargamel's brother or something and that whatever "it" was, it would surely spell doom for the smurf way of life.
A childhood belief about misheard lyrics

others believed things like this
- I thought Maria Carey's 1998 ballad was 'I'd give my arms' instead of 'I'd give my all'.— Briony Maggs
- i thought I got one year older when my family sang the birthday song.— I am the ghost
- I was told that for every christmas carol i sang before christmas time santa would give me one less present!— K
- I used to believe that the lyrics to Bon Jovi's "You Give Love a Bad Name" were "You give love a Band-Aid".Anon
- My brother was listening to The Corrs and instead "leave me breathless" he would sing "give me breakfast".Anon
- When Madonna sang "let the choir sing" in "Like a Prayer", I thought she was singing "love by proxy"— Kristen