About.com writes: “Mario 2 was not really a Mario title at all, but a game released in Japan as Doki Doki Panic. Nintendo had decided the actual Mario 2 was too difficult for an overseas release and pulled the title and modified Doki Doki instead. In order to make Doki Doki Panic Mario-like, they made some small changes, upgraded the character graphics, and sent “Mario 2″ onto the market.”
[via Joystiq]