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Holding A+B while starting the game will display a hidden message: "KONAMI ONE GRADIUS ONE"

This appears to be a diagnostic screen, as some people have reported that it may say "TEN" instead of "ONE" on one line or the other. It is theorized that this screen displays the result of some kind of code verification algorithm or checksum, with each line representing the status of a different portion of the game's code.
Contributed by Spherix
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A special version of Gradius was given away by ArchiMENdes, a ramen noodle company. The Vic Viper would power up using cups of ramen noodles rather than the usual pods.
Contributed by Psychospacecow
While porting the game to the NES, one of the developers snuck in a code that gave you all the powerups, due to his inability to play the game. He then forgot to take the code out.

The code later became known as the Konami Code.
Contributed by ummwat