-SEEMS TO HAPPEN WHEN YOU PERFORM A VERY WEAK THROW more often than not
I have a crossbow, haven't tested it without an item on my back in primary slot
Picked up a CRZ73 rifle. Threw the weapon, it landed and then disappeared.
I have done this twice now. Was going to keep testing but have an invisible zombie below the ladder I'm cowering on, so I ain't going down until the server resets :)
edit- happens with mp-133 shotgun