This game has unused graphics.
Flappy Bird is one of the most addicting and annoying games made for smartphones. It was removed from the App Store and Google Play on February 10, 2014.
There were apparently going to be red pipes scattered around during the game, but only green pipes are used. Two red pipes still exist in atlas.png, but the second is a copy of the first (as opposed to being flipped like the green ones). The atlas.txt file refers to the two unused graphics as pipe2_down and pipe2_up. They can be added to the game by replacing the numbers after pipe_down and pipe_up in the atlas.txt file with the numbers after pipe2_down and pipe2_up respectively.