Items held cannot be seen by other players. Items dropped cannot be seen by other players. Items transferred from backpack to backpack are not transferred properly.

So I had a Hunting backpack in my hands to give to a friend. When I met up with said friend, he says he does not see the backpack I was holding. I place it on the ground and he still could not see it. Since I was also wearing a Hunting backpack, I instead dropped my backpack to the ground which he was able to see. So I switch items I was carrying from my backpack to the other backpack. I take and wear the supposedly "bugged" backpack which now has my items. When he goes to my original backpack he says my items are still there. I told him to pick it up and wear it anyway. We thought we actually duped my items, but that was not the case: whenever he removed items out of my original backpack, items from my backpack was also removed. I relogged and that seemed to removed the "ghost" items that were in his backpack.