When I was in my early twenties (unfortunately, that was about 15 years ago), a friend of mine and I were huge fans of Jim Morrison and The Doors. One day as we were listening to the song "L.A. Woman", I sang along to the line that goes, "L.A. woman, Sunday afternoon". My friend was very surprised and told me that he had thought for the longest time that Jim was singing "idiot woman gonna have to do"! This was funny to me because 1) my friend was an even bigger fan than I was (and would seemingly have known this lyric, as it contains the song title; and 2) because Jim repeats that line several times in the song.
A childhood belief about misheard lyrics

others believed things like this
- I thought the line in the song "more than a woman" was "four-legged woman".— boo
- I used to think that the song "Evil Woman" was "Medieval Woman". Anon
- My sister just recently realized that the lyric "more than a woman" wasn't 4 letter woman!— Anne
- I always thought that Zuccero's song "Senza una dona" was actually "Sence of a woman"— Miroslava
- In the song "We're going to Ibiza", my friend thought they said "we're going to eat pizza"!— Smaranda
- I thought the song "Party Time" by Moby was actually saying "heart attack" (this was before I knew the title)— Elviessa