I have two really good ones:
Elvis' "Return to Sender"...I thought he was singing "Return Lucinda"
We Built This City on Rock and Roll? Uh uh....we'll both get skinny on rock and roll.
A childhood belief about misheard lyrics

others believed things like this
- I thought that the song "Built this City" by starship went as follows. "Milk and Cereal, Rock and Roll"Anon
- I thought Rock and Roll were 2 musicians, with one who rocked and one who rolled.Anon
- My friends and I used to believe that if you planted a rock, it would grow a rock tree. Anon
- I used to believe the the Elton John song, "Crocodile Rock" was about a Crocodile, named Rock.Anon
- When I was young, I really used to believe that 'R.I.P.' (on a gravestone) meant 'Return If Possible.'— Colin
- I Used To Believe In The Scab Fairy, That She Would Pay You In Return For Those Scabs!— KellGirl