Enter World 5-7 as anything but small Mario and with no Starman power. Bump the first wood block you see to release a Fire Flower, but don't grab it just yet. Hit the first Question Block in the group of three to the right, and the Fire Flower will start to flash. Next, go forward just enough to make the Pile Driver Micro-Goomba attack. Take damage to downgrade to small Mario, then grab the Fire Flower. Instead of transforming to Super Mario, you'll become Fiery Mario!

The block you hit that makes the Fire Flower flash would contain a Starman if you entered the level with Starman power, so something about hitting the block without Starman power while the Fire Flower is exposed causes this glitch. This glitch also works in World 7-3 and with the Super Leaf (though it won't flash) in World 6-1.