The bug

If you look into a container, which is a chest, ender chest, trap chest or shulker box, in spectator mode and hover over an item with your mouse and then press F, you will have a ghost item in your offhand when you go back into another gamemode. This item can’t be removed from the offhand using F. If you click on the item in creative mode, it will become a normal item, if you click on it in survival, it will disappear.

How to reproduce:

1. Place a chest
2. Take any block
3. Go into spectator mode
4. Look into the chest
5. Hover over an item in your inventory
6. Press F
7. Go back into creative mode
   You will have a ghost item in your offhand, if you place the block, it will instantly vanish.