Early on in Sakuya's stage in Mega Mari (a Touhou-themed clone of Mega Man), there is another hallway underneath the floor. A Lyrica is down there as well as a ladder leading down, but there is no way to access it.
Double Dealing Character have swimming sprites for Wakasagihime that are never used in the game itself.
The Arcade GameAstro Blaster had a Sector 8 which would appear in the Attract Mode if that was left to run long enough, but can't actually be played because the game loops after Sector 7. This made Secret Bonus Number 20 impossible to obtain without hacking the game.
Astrosmash for the Intellivision was programmed as a compilation of two asteroid-shooting games, Avalanche! and Meteor!. Since Meteor! was a simple clone of Asteroids, Mattel's lawyers ordered it removed, but its code remained in Astrosmash, and a rare glitch could cause Meteor! to load when resetting the game. Meteor! was later reprogrammed to be less Asteroids-like and released as Space Hawk.
The cutscenes in Thwaite were originally supposed to have characters walking around, like during the game. Graphics for walking characters at cutscene scale are still present in the tile sheet.