I'm absolutely stunned someone else thought, like I, that London Bridge Is Falling Down was London Britches Falling Down! I sang it that way until i was 18!
I don't know about this one but I until now just realized that the Bee Gees song "You Should Be Dancing" I thought the line was always "What you doing on your back?" :/
A childhood belief about misheard lyrics

others believed things like this
- up until about 6 years ago i thought everton was in london!Anon
- When i was little i thought casualty was a big hospital in londonAnon
- When I was little I thought the london underground was a terrorist organisationAnon
- I used to believe that falling leaves were dead butterflies, and I always buried them in the back yeard.— waters
- Did anybody besides me believe that Londonderry Air was actually London Derriere?— Texas granny
- I used to believe that london, paris, venice, and switzerland were all cities in France. — an american...