This happened to me several times. there is no specific item that seems to trigger it but I have a habit of checking my inventory window (left side after hitting TAB in game) so I can see if there are items inside pockets of clothes or barrels. the window then shows nothing. I have tried to scroll up nd down but nothing. when this happens nothings shows in that window for the rest of the game including checking zombies I killed for loot. but wait there's more! typically when this happens I will not be able to return to the same server after I log out.