Within the game's files is code for a 6th difficulty that goes unused. It would've been called Encore, and "The Disappearance of Hatsune Miku" is the only song that has a fully completed chart for it. Encore would've been the hardest difficulty in the game, as it would've contained it's own unique notes in the form of randomization. Meaning that the notes (Triangle, Square, Circle, and X) would be different each time the song is played. The only exception to this would've been if the player chose to retry the song.
Contributed by ShyanVixen