Super Guide Block
From the Super Mario Wiki
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A Super Guide Block is a special green ! Block that appears in the single player modes of New Super Mario Bros. Wii and both releases of New Super Mario Bros. U. It will appear when the player has lost multiple lives consecutively in the same level. If Mario hits the block, the Super Guide mode will be activated, Luigi will appear, and the computer within the game controls him to clear the stage in place of the player.
New Super Mario Bros. Wii
In New Super Mario Bros. Wii, the Super Guide Block appears when a player has lost a life eight times in a row on one level. If the player presses on , the player may choose to deactivate the Super Guide mode and take control of Luigi. However, if the level is beaten, it does not count as an actual win. Items collected do not count as well. If it appears at all, the save file's stars will no longer be shiny.
New Super Mario Bros. U / New Super Mario Bros. U Deluxe
The Super Guide Block returns in New Super Mario Bros. U, working as it did in New Super Mario Bros. Wii. In this game, only five lives need to be lost, rather than eight. In addition, save file stars will only lose their shine if the player hits the block.
This feature returns in New Super Mario Bros. U Deluxe, acting identically to its appearance in the original Wii U game. However, when playing as Luigi in New Super Mario Bros. U, the block is colored red and summons a computer-controlled Mario in place of Luigi.