1. Outfit a character with a backpack and no other clothing that have storage.
2. Put a pistol and a compass in the backpack, then fill it with any 1 slot item that can be held in the character's hand.
3. Drag the pistol, compass and 1 slot item to hotbar.
4. Select the compass with the hotbar.
5. Select the pistol with the hotbar.
6. Observe how the compass took the pistol's place.
7. Select the 1 slot item with the hotbar.
8. Observe how the character drops the pistol on the ground.

[Note] The expected result could also read, ". . . the item currently being held will transfer to it's last known location in the player's inventory."

[Note 2] Any items can be used to reproduce this effect, the ones named in the 'to reproduce' section were just examples.

[Note 3] Any player not aware of this specific inventory mechanic would very likely unintentionally drop their item.