This might work in all similar situations, but an good place to try this is World 1-1. Near the beginning of the stage, there will be two Goombas walking down from a high platform onto another platform with a Question Block in the middle. Run underneath the platforms until the lower one is about half a block width away from the left side of the screen, then turn around and run back to hit the left-most block from below as the last Goomba walks over it. The Goomba will turn into an upside-down green Koopa shell as he dies! If you knocked the Mushroom out of the Question Block as you ran by the first time, the shell will be red.

A somewhat easier place to pull this off is World 6-1, in the area with the hidden 1-Up Mushroom (see screenshots 3 and 4). Leave about half a block between the left side of the screen and the block that you'll bump from below, and you can consistently turn Spinies into Koopa shells. Since Spinies survive knocks from below, the Koopa shells that appear will also survive, and will turn into live Koopa Troopas on the ground. It's interesting to note that the red Koopa Troopas made with this glitch can walk off of platforms, unlike the normal red Troopas.