Here's a cool bug for the Japanese version of SMBDX. As Fiery Mario, hit an enemy. While Mario is powering down, press Start to pause the game, and then choose "save." When you end the game and restart your saved game, Mario will either be (depending on your timing) small, Fiery, or small and Fiery! Unlike the small and Fiery bug in the original SMB, Mario's fireballs will shoot out from above his head. Since the game considers Mario to be "small," one hit will kill him. Collecting a Super Mushroom will make Mario grow, and he'll still be in his Fiery colors, and he'll still be able to shoot fire.

This bug was made possible by a change in the Japanese version of SMBDX, where Mario retains his form when you save your game.