Various websites over the interweb, including 4cr have uncovered a cheat for the highly popular New Super Mario Bros. The game that has practically sold out across the globe.
"Gamers who’ve already finished the game will be able to access this secret Challenge Mode by hitting Start in the world map and pushing the following combination of buttons during the Start Menu: L R L R X X Y Y"
This enables "Challenge Mode" which alters the game only slightly. With Challenge Mode enabled, you will no longer be able to pan backwards in the level, much like the original Super Mario Bros. on the NES.
While its nice to see hidden modes and extras, I can't help but feel a little let down by this one. C'mon Nintendo, is that the best you can come up with?