If you manage to get Mario into a cannon hole when he has no power left, you'll still be able to shoot him out... but he will die as soon as he lands. An easy place to perform this is in Course 2: Whomp's Fortress. Deplete the power meter to three pieces or less, jump in the cannon, and aim for the corner on the first wall sticking out. If you hit high enough off the corner, Mario will land back into the cannon with no energy.

Another good spot is in Course 8: Shifting Sand Land. Jump in the cannon with at least half of your power meter depleted, aim as far up and left as possible, then shoot out. Mario will hit an invisible wall and fall back into the cannon with no power remaining. If you had a Wing Cap on, you can fly around the course with no power!