Player in spectator mode can recieve an item in offhand by clicking 'F' on item in any opened container menu (etc chest, furnace).

Steps to reproduce:
1. Place a chest
2. Put an item in chest or in your inventory
3. Go to spectator mode
4. Open chest
5. Press 'F' on any item
6. Close chest
7. Open 'E' inventory
8. You have an item in offhand

This item is phantom - if you click on it in spectator in your 'E' inventory, it dissapears. However changing gamemode makes it real.