So, this happens when you usually have a item that takes more than 1 slot. It happens really rare, but it kinda annoys people. So when you log out of server, its all fine, then you come back. Like 1 or 2 items from your backpack are lost because that thing that takes more than 1 slot ( for example medkit ) takes 4 slots and sometimes while game is still loading, it covers slots and it does not allow other items to be placed in backpack. So it covers that slot, and game fails to find anthoner slot for that item and item just gets deleted.