(1) So I take out a pot from backpack no.1 or no.2. ( now have free space)
(2) then I pick up that dropped item. (full again)
(3) Then, the strange thing is I can pick up the pot again in spite of full backpack.
There should be no free space for that pot. By this way, I can use one, sometimes two, extra free slots that is not visible.

BUT . . . Everything is normal when . . .
(1) I take out tha pot(s) from backpack 1 or 2
(2) Switch between the backpack no. 1-2-3 (the free slot is now at the backpack no.3)
(3) pick up dropped item ( full again)
(4) then, can't pick up the pot again.
Sorry for my way of explanation.
And I'm not really sure it is a bug or not.

