In the SNES game, when Mario or Luigi gets hurt while having a power-up (i.e. fire flower or cape), Mario and Luigi will become Small Mario/Small Luigi, as they did in Super Mario Bros. In the Game Boy Advance game, Power-up Mario/Power-up Luigi will become Super Mario/Super Luigi when they are hurt, as they do in Super Mario Bros. 3. In both versions, if Super Mario/Super Luigi get hurt, the reserve item in the box at the top of the screen will drop down.
Contributed by Kakariko Kid