Sometimes when you find fruit such as a kiwi or orange on the ground and eat it, it then takes up a space in your inventory such as a backpack or pants, but it does not show up. So it looks like nothing is there, but there is a fruit with 0% left there making it so you can't put anything in this space. If you log off and log back into a server it will show up visibly as a fruit with 0% there and you can now get rid of it and use this space again.