To reproduce with a weapon:
Pick up a primary weapon (rifle, for instance). Ensure that it is not in hands (is on back).
Pick up another primary weapon into hands.
Drop the second weapon from the hands using the red X.
Wait.
The weapon will appear at the bottom of the inventory.
To reproduce with Gas Canister:
Pick up a Gas Canister into hands.
Drop with red X.
Wait.
Gas Canister will appear at top of the inventory.