The Minstrel Boy

In the episode titled "The Minstrel Boy" Duncan Hammer sings what he later claims to be "The Minstrel Boy," as he noted in the season finale. As a ghost he even says, "how would you know the lyrics to The Minstrel Boy."

The problem is that he didn't sing the lyrics to The Minstrel Boy, he actually sang the song "The Son of God Goes Forth to War." The latter uses the same tune, but it isn't technically The Minstrel Boy.