Throw a Potion on top of a grass clump (veggies and shells work best) or a POW Block. Enter Sub-Space, then press Up again to exit through the door. When the colored scenery is about to reappear, quickly and repeatedly press B. With the right timing, the item you pulled up will be on your head. You can't throw the item, but if you enter a door or climb to a new area, the item will become real and you'll be holding it like normal. If you lose a life while carrying the illusion item, you'll have a real item in your hands when you retry the stage!

The illusion item glitch won't work with 1-Ups, Stopwatches, or Rockets. You can get an illusion Mushroom Block, but it will disappear when you enter a new area. An illusion Potion in World 6-1 leads to the warp backwards glitch...

Illusion enemy: A bomb or Bob-omb as an illusion item will explode in your character's face, but pick up an enemy before the bomb explodes and you'll end up with an illusion enemy! Like some illusion items, the illusion enemy will become a real enemy in your hands when you enter a new area.

Rocket glitch: Though you can't get an illusion Rocket, you can cause a neat glitch by pulling up a Rocket while you're carrying an illusion item. The item will stay in mid-air when you blast off, and the upper-right section of the Rocket will be missing.