Super Guide Block

A Super Guide Block is a special green ! Block that first appears in the single-player modes of New Super Mario Bros. Wii and both releases of New Super Mario Bros. U. It appears when the player has lost multiple lives consecutively in the same level and is playing alone. If Mario hits the block, the Super Guide mode is activated, Luigi appears, 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, with the block emitting a doorbell sound to alert the player of its presence. If the player presses on, the player can 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 and the player saves the game, the save file's stars [[Missable content|are no longer shiny and there is no way to get the shine back unless you make a new save file.

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 lose their shine only if the player hits the block, but the stars' shine will return if the same course is cleared without using the block.

This feature returns in New Super Mario Bros. U Deluxe, acting identically to its appearance in the original Wii U game, though with a new sound effect. When the player controls Luigi in New Super Mario Bros. U, the block is colored red and summons a computer-controlled Mario in place of Luigi. In addition, after the player uses the Super Guide Block, the stars on the player's save file remain shiny.

Trivia

 * When Luigi or Mario completes the Super Guide, he says, "Hmm, okay," rather than their respective standard quotes, "Phew, made it!" (Luigi), or "Oh, yeah! Mario time!" (Mario).