Item Vanish

From SmashWiki, the Super Smash Bros. wiki

Item Vanish is a phenomenon that originated in Super Smash Bros. Melee and returns in Super Smash Bros. Brawl.

It is performed mostly by accident when a character is on the ground and is attempting to grab/catch a disappearing item. If the timing of the button press to pick up the item is perfect, the character will begin to grab the item (that is, begin the pickup animation) yet the item disappears before it can actually be grabbed. This can happen with any item that can be picked up, including heavy items. This is how a SLAPASTICK works.

The same effect can happen if the item falls out of reach (say, due to a destructible platform) before it can be picked up, or another player grabs the item first.