When I was young I used to think the line 'She wrote upon it.' in Elvis's 'Return To Sender' was actually 'She rode a pony.' I still sing the wrong line to this day (although only with happy irony) because it sounds so much like it!!
A childhood belief about misheard lyrics

others believed things like this
- I used to believe that a "punch line" meant a line of people waiting to dip punch from a punch bowl.— Sheila
- When I was young whenever I rode on an airplane I always thought we were traveling to another planet. — Dennis Rueda
- When I was young, I really used to believe that 'R.I.P.' (on a gravestone) meant 'Return If Possible.'— Colin
- i used to belive that you drive your car on the line in the street.— abcdef
- I used to believe that child line sold childrens toys— Gayle
- I used to believe that the last line of Row Row Row Your Boat was 'Life is butter dream'.— Mindless Misfit