Phalanx

Boot the game with the Opening Disk. After the intro is over, at the NOTICE screen with a chibi Mel on the bottom-right, hold the following keys on the keyboard: Y + U + K + O. If done right, her skirt will fall off.

Boot the game and at the configuration screen, press the following keys. However, for them to work, you have to first press and hold the first key, then hold the second one (while still holding the first one), then release the first key and hold the third one, then release the second one and hold the fourth one, etc. So for the invincibility cheat you would hold S + U, then U + G, then G + O, etc.