Inventory bug
Sometimes when I move things around in the inventories, they overlap.
They are visibly on one slot overlapping.
e.g.: I have matches and apple (at spot A).
I move apple to different spot (B), and then I wanna put matches on the spot A. It's red and doesn't want to go there.
Then when I put matches where I SEE apple (spot B), it will let me put them there.
When I move apple again (on spot C) and put the apple on the spot A where I couldn't put them before, it will be possible as apple "unbinds" from slot A.

Stash bug
Something similar happened when I was burying a dry bag backpack, i had the backpack and a captain hat on the same spot, and when I dug up the bag, it appeared to be under the items (I could take the captain hat and I had like some magazines and weapon accessories under it).