In 2021, several cheat codes were discovered in the Game Boy version of the game, the first two of which must be activated through the title screen.

•Ending and Credits: Up/Down/Left/Right/Select/Start.

•Sound Test: Down/Up/Down/Up/Select/B.

•Level Select: While paused in normal gameplay, press Down/Select/Up/Left/Select/Right/B. Press A or B to select a level.

Hidden Tribute to Hiroko Kasahara: To access it, you must first access the Sound Test menu. Then, set the BGM/SE index to 02 and press A once, then set BGM/SE index 19 and press A once more. Lastly, you must input the following lengthy code:
Up, Up, Down, Down, Left, Left, Right, Right, Select, Select, Up, Up, Down, Down, Left, Left, Right, Right, B, B, Up, Down, Left, Right, Select, Up, Down, Left, Right, B, Select, Right, Left, Down, Up, B, Right, Left, Down, Up, Select, Select, Right, Right, Left, Left, Down, Down, Up, Up, B, B, Right, Right, Left, Left, Down, Down, Up, Up, Start.

If inputted correctly, an alternate credits sequence will play out. The normal credits plays the game's ending theme and lists the name of programmer Yutaka ★Hiroko★ Sakashita among the game's developers. This alternate credits sequence instead plays the Vs. Mode music and lists the name Hiroko Kasahara at the top of the screen. Kasahara is a J-Pop singer, and the names of the game's developers in the credits are replaced with a comprehensive discography featuring the names of several albums she had released up to 1993, including the release dates for her then-upcoming 1993 albums "L'Express Fantasie" and "Nature". Considering Sakashita used Kasahara's first name on the standard credits sequence, he is most likely the one who included this Easter egg to document his love of her music at the time the game shipped. The fact that this sequence is hidden behind two cheat codes that would take almost 30 years to be discovered, with one of them being too long to easily guess, suggests that it was never meant to be seen by players.
Contributed by MehDeletingLater