When using commands to readd an item you just used to your the slot you just emptied, a ghost item appears in that slot until you enter your inventory and click on the slot.

Steps to Reproduce:
1. Run the command "/scoreboard objectives add grassUser minecraft.used:minecraft.grass_block" in chat.
2. Place a two block command block chain like depicted below.

3. In the top command block, type "/scoreboard players set @a grassUser 0" and set it to Always Active

4. In the bottom command block, type "/give @a[scores=\{grassUser=1..}] minecraft:grass_block" and set it to Always Active Note: Remove that random backslash in the command I can't remove it on the issue tracker for some reason.

5. Switch to survival mode and place a grass block. You now have a ghost item in your mainhand.