Super Mario Bros. 2. For those of you unfamiliar with the history of the game, you might have wondered why that game seemed completely different from the first Mario game. Well the reason for that was the real sequel was considered to be too similar to the first one, and too difficult so it was never released outside Japan. Still, Nintendo were stuck for a Mario sequel, so what they did was they took an existing game, Yume Kōjō: Doki Doki Panic, and simply substituted those characters for Mario characters.
Now the game itself isn't necessarily bad, but it shouldn't be considered a true Super Mario game.