Jingle bells...
I used to believe until I was about 22 that in Jingle bells it was a "one horse soap and sleigh". My husband politely informed me it's "one horse OPEN sleigh..."
I never really thought about it but was so very embarrassed when he told me what it really was.
*duh* PFFFFTTTTTTTT!
A childhood belief about misheard lyrics

others believed things like this
- I used to think the song "Walk this way" by Arosmith went horse and sleigh insted of singing walk this way.— Derek
- When I was three I used to believe that cow and horse are wife and husband.— xenia
- I used to believe that The Horse was The Cow's husband, and that it produced chocolate milk.— Laura
- I used to think that beer was horse pee. Probably because that's what my mom told me...— Kara
- I was always told that beer was made from horse pee.— Just me
- I believe there are horses making the car go fast when people told me the horse power of a car.— Jake