Mickey Mania: The Timeless Adventures of Mickey Mouse/Hidden content
From Sega Retro
On the sound test screen, set Music to "Continue", FX to "Appear" and Speech to "Think....". Highlight the "Exit" option and hold 
for five seconds. A jingle will play if successful, and the level select screen will appear when attempting to start a new game.
The Japanese version of Mickey Mania is region locked, meaning that without modifications to the Mega Drive hardware, it cannot run on an NTSC-U or PAL machine. As with other region locked games, a message is displayed when the user attempts to play this cartridge on the wrong system, however if the region is changed to NTSC-J when this screen is displayed, the game will react and boot normally, without the need to reset the console.
Programmer Jon Burton added this feature as an Easter egg, as his Mega Drive was modified to allow region switching.[1] It was not discovered by the public until region changing options were built into Mega Drive emulators. This the only known commercial Mega Drive game that will allow you to bypass a region lock screen this way.
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