When in Creative Mode, if you throw items out of the inventory, and then open a container (chest whatever) without opening the inventory before, then the items will appear back.

To Replicate:
Throw Items from inventory onto the ground. (pic1-3)
Do NOT open inventory, and open a chest. (pic4)
Items will suddenly reappear in inventory.(pic5)
The other items will still be on the ground in the background.

I searched alot and couldn't find this bug anywhere, so I hope it hasn't been already reported.