You've activated Hard Mode! In Hard Mode, Mario will die any time he takes damage, and the timer will be more strict. So treasure your Yoshi, and hold on to your hat, you're in for a wild ride!
”
~ In-game message upon activating Hard Mode.
Hard Mode is a secret found upon pressing A, B and + on the Star Coin menu after fully beating Newer Super Mario Bros. Wii and Newer Super Mario Bros. Wii Plus. In this mode, the player will instantly die upon taking damage, even as Super Mario. The timer is also much shorter, which just adds onto the challenge. This mode doesn't count towards anything, so its just a little neat bonus.
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on the Star Coin menu.
