My bag looks like it has an empty slot still, however the number on the bag says 16/16 -- it thinks it's full already. But I can open and close it and one slot clearly 'looks' empty. I can't move items into the 'empty' slot, nor can I drag anything out of the 'empty' slot.
It seemed to occur around the time I had two auctions expire without being bought - although I couldn't guarantee the timing, it's just that I noticed it slightly after that occurred.