Harpua
"Harpua" is a Phish song that was a staple of their live concerts, but has now become a rarity and has only been performed live. Though it always follows the same basic structure (see The Divided Sky and You Enjoy Myself for other examples of songs in which variation occurs within a fixed structure), Trey Anastasio always narrates a different story at every concert in the middle of the song.
Read more about Harpua.