There is a Super Mario Bros. reference found throughout the game that is easy to find. Take a close look at the pipes from which the small flying enemies appear from. You will notice that they are the same design of the pipes from of the Super Mario Bros. series. The green pipes that you find most closely resemble the Super Mario Bros. pipes. Be careful when looking at the pipes, as the flying enemies that leave them do not stop appearing. Shoot them before they hit you.