This can be unlocked on any difficulty but the higher the difficulty the quicker you can fail out
The shortest song in the game is Mississippi Queen (Second tier or Career) and on expert its easy to tell once you are passed 90% of the song as 90% is just after the third triple chord (Green, Yellow and Blue)
The song is easy enough that any medium player can manage it.
As soon as you are passed the third triple chord, strum (Or press the buttons on the controller) fast to fail the song.
This can be unlocked on any song but this song is my suggestion as its the shortest
Here's a video demonstration on expert(I was using a 360 controller)