When in creative mode, if you try swapping an item using the number keys from the hotbar to another spot on the hotbar, then the item that you moved is replaced by the item in the slot it was moved to, or air if the slot was empty. The client does not update when this happens causing a ghost item to appear in the inventory which when updated changes to the item it actually is.

Reproduce:
1. Place cobblestone on your hotbar in the first slot
2. Place smooth stone on your hotbar on the second slot
3. Open your inventory and press 1 while hovering over the smooth stone
4. Close your inventory and try to place the smooth stone that is now in slot 1, you will notice it is replaced with cobblestone but the cobblestone that is now in slot 2 does not change to smooth stone when updated meaning the smooth stone has been deleted.

This happens with most blocks and items, including swords, buckets of water and so on.