Sometimes when changing one item for another on ground, or when you start eating while holding smth in your hands. Item you held or tried to pickup will either disappear completely or become unpickable by any means. It just changes it's icon to blank and becomes unpickable, still interactable by some means.

Cases:

• Just today on stable 0.60, held Sedan wheel i've picked up in a garage near building in which i've drank Soda can, character dropped wheel on the ground as always, and then Soda can just froze in my hands AND inside some clothing on ground, i've removed Soda can manually from hands, but wheel just disappeared. Reconnected to server immediately and got teleported to barrel from which i've picked up that wheel, still no wheel neither in barrel nor in a building. --- Update: Just got another wheel to disappear in about 5 minutes left on ground, so maybe it's just CLE acting silly.
• Several cases of 5.56 magazines becoming unpickable, but still allowing to empty them of ammo, both on 0.60 exp and current stable. Happens just when you try to swap magazine in your inventory for magazine on the ground. Nothing happens first, when you try again - magazine on ground loses it's icon, nothing happens ever after, reconnects not helping.