This is somewhat a duplicate of MC-41113, with a different methodology and issues.

No apparent risk of item-duplication or other exploits.

Ghost Items can be created by loading a full hotbar (Default: 'x' Key+ <num> Key) and using the Pick Block button (Default: Middle Mouse Button) repeatedly

• These blocks will not disappear with /clear or by Shift-Clicking the "Destroy Item" slot in Creative Mode.
• The following will remove these items:
  1. Opening a different type of inventory GUI will remove these items (e.g. Opening a Chest)
  2. Relogging
  3. Killing the Player
  4. Filling the inventory with real items (items that are picked up or given using the /give command will override "Ghost items")
  5. Changing Game Mode to Survival and attempting to use the "Ghost items"

Expected Behavior: "Pick Block" with a fully-loaded Hotbar should not create "Ghost Items" within player's inventory.

How to Replicate (Creating the "Ghost Items"):

1. Utilize the Load/Save Hotbar feature such that at least 1 hotbar will filled with all slots (i.e. Save a hotbar with all 9 inventory slots filled with items).
2. Load a hotbar.
3. Use the "Pick Block" button (Middle Mouse Button) to select a block that does NOT exist in the hotbar (For example, "Pick Block" a Grass Block while the hotbar is full of random redstone components)
4. Repeat steps 2 and 3 repeatedly.
5. Open your inventory. At this point, you should notice your inventor is full of additional items (besides the items on your hotbar).
6. Attempt to remove these items (while in creative, and not in any particular order)
   1. /clear
   2. Shift-Click the "Destroy Item" slot in the Player's Creative Inventory Menu