Using hotbar keys can sometimes create Ghost Items.

There is 2 ways to do it, both in creative mode :

First way

1. Put an item in one of your hotbar slot, and make sure it's not a ghost item. Select this slot. For the example, I will take a diamond sword and place it in my first slot, and I will select this slot.
2. Open the inventory. Go in the tab of the selected item. Here I will go in the weapon tab.
3. Take the item, and destroy it by putting it in a random slot from the creative inventory.
4. Now, hover the item you chose. Now, press the key from your keyboard's hotbar wich correspond to the slot you selected. Here, I will hover the diamond sword and press the "1" key.
5. Close your inventory. The item is now a ghost item.

Second way

1. Same step than for the first way.
2. Open your inventory. Hover the item you selected in your hotbar, then press a key from your keyboard to switch the item on another slot of your hotbar. Here, I will hover the diamond sword in my 1st slot and press the "2" key.
3. Close your inventory. The item is now a ghost.

Check the attachments for videos.