SPECIAL CONDITIONS
A hidden 1UP block will appear in the first level of any world
IF
it is the first world you are playing (not always world 1, since you can start from any world in game b mode)
or
you entered the world from a warp zone
or
you are continuing after a game over screen ( hold 'a' while pressing start at the title screen to continue from the first level of the world you lost your last life in)
or
you collected every coin in the third level of the previous world (except 1-3, where you can miss 2 coins)
AND
you have not previously died on the level since reaching the 16th floor tile to the left of the 1UP block, with the tile directly underneath it counting as 1