Mondegreen, n., the misunderstanding of a word or phrase that gives it a new meaning. This occurs frequently with song lyrics.
A few examples: during catechism, some of us misheard the phrase “Cant (or Chant) as you can, not as you can’t” as “Can’t as you can, not as you can’t”. I found it encouraging when thinking about fasting—do what you can. I was amused to see that the latter half of the phrase “blessed art thou amongst women” was misheard by Irish boys as “a monk’s swimmin’”. My mom tells me that the hymn “Lead on, O King Eternal” was often sung “Lead on, O kinky turtle” as her class hymn in college.