If you are carrying two guns (one on back, one in hands) and you perform an action such as drinking a PET bottle, or eating some beans, the gun in your hands will disappear.
After you have completed eating or whatever, that item is bugged. It does not function and sometimes lingers in the inventory.
Once the action has completed, your character will never pull out the second gun again..
The item is not dropped on the ground or replaced with the weapon on your back, it is lost forever.