In World 5-7, go down the pipe guarded by a fire-spitting Piranha Plant. In the next room is a white block on the ground, with various blocks to the right leading up to a pipe. Duck down on the white block for five game seconds, and Mario will drop behind the scenery. Before Mario returns to the foreground, hurry up to the pipe that leads back up top, and the enemies there won't be able to touch Mario. The invincibility will last until you enter a pipe or exit the stage.