This trick is good to try in World 2-3 after beating Tryclyde, where you'll have a doorway and lots of Mushroom Blocks to work with. Put one or two Mushroom Blocks horizontally on top of the door. Pick up another Mushroom Block, and as you're moving, try to throw the Mushroom Block into the bottom corner of one of the Mushroom Blocks sitting on the door. If you hit the right spot, it should disappear into the Mushroom Block sitting on the door. It's pretty easy to reproduce this glitch once you successfully pull it off the first time.