R-Type Cheats

In the stage with all the green cells (either Level 4 or 5), look for a section about 45 seconds in where the top border suddenly has a gap about 3 ship lengths long. Fly into the gap and move backwards. If you're quick and the back edge doesn't scroll off screen, you will teleport to a bonus stage. As a note, the boss of the secret stage doesn't flash when he's hit, unlike all the others.