I messed around in my inventory while playing the experimental version. During one of the server laggs items in my inventory got duplicated. The duplicated item seemed to have the same identity since both items get highlighted when drag&dropped around the inventory. Hence I called it a "Ghost-" Item.
I had a suspicion that it was related to server lag / losing connection to the server and regaining it. After experimenting I confirmed my suspicion. {F32814}