Released: 1996, SNES
Super Mario RPG: Legend of the Seven Stars is half-RPG, half-Super Mario Bros. Although it sounded like a blasphemous mix at the time, headed by Nintendo and RPG giant Square (then at what many people consider their peak) the results were spectacular, flawlessly blending two genres into a new one entirely: the "Mario RPG". A legacy that would be continued with Paper Mario, Mario & Luigi: Superstar Saga, and several others.
But it seems that development of this game was a rough process, as a number of areas were left out (and sadly, in most cases, the tilesets were altered to the point where these areas are a garbled mess, or were deleted entirely), and several enemies - presumably intended for these cut areas - remained unused.
If you're a fan of debug modes, Super Mario RPG actually features two of them, as well as a debug room and even several debugging items!
All in all, there are some extremely intriguing things hiding in the game...