I used to think one line to the All American Rejects' "Gives You Hell" was "I've never seen you fart so hard." Turns out it's actually "fall!"
Also, I thought that some Mamma Mia song had a line that said "Ooh, since the baby farted..." My friend agreed with me. After a few more times hearing it I figured out that it's actually "Since the day we parted."
I have a pretty wrong mind when it comes to lyrics... and farts...
A childhood belief about misheard lyrics

others believed things like this
- I used to believe that if you farted really hard, you could fart a hole in your pants. How dumb was that?— stargirl
- Pink Floyd- Brick Wall.
I used to believe it was "we don't need no fart control" in the second line there.— Norwegian Dude From Kopervik - I used to think that the song"Stayin Alive" said "Staying In Line" like at school when I was little. — MacKenzie
- I used to think that the line in the song "Life in the Fast Lane" was "Fights in the Bat Plane".Anon
- i thought the line "saving my face" from the KT Tunstall song of the same name was "shaving my face"Anon
- I thought the line in the song "more than a woman" was "four-legged woman".— boo