Throw down a Potion on top of some grass. Enter Sub-Space, then press Up to exit. As soon as the door opens, quickly and repeatedly press B. With the right timing, the item that you pulled out will be on top of your head. You can't throw it, but if you go in a door, or climb up to a new area, the item will become real and you'll be holding it like normal. If you get killed while carrying the illusion item, you'll have a real item in your hands when you retry the stage!

The "illusion item" has a strange effect on Rockets. Set up the illusion item glitch, then pull up a Rocket. When it flies upward, your item will stay where it was, and the upper right corner of the Rocket will be missing.