Debug Mode:
To enter a special debug mode, wait for the Core Design copyright message to appear, then press Up, Down, Left, Right, Down, Down, Right, Right, Triangle. You will be taken to a test menu where you can choose your starting level, maximize your firepower, view all the videos, see the credits, & improve the games appearance.

Invincibility:
During the game, press Start & Select to quit. At the title screen, press Up, Up, Up, Down, Down, Down, Right, Right, Triangle. A chime will play if entered correctly. Now start the game & your energy bar will go down as normal, but will slowly rise again.