I call it the inventory holes.
When you're in a store and have your inventory open (with the paperdoll) and right click an item, that item gets sold and swapped with whatever you are using on your paperdoll, and that sold item's slot gets permanently (until log out, that is) filled by "nothing."
But wait! There's more!
When you get new items, and arrange them, these "holes" count as filled slots, so you could have a bunch of swapped out equipment that fills up your inventory and you won't be able to pick up new items. Basically, you're reducing the size of your inventory unless you exit the game and start anew.
Found this out, thought nothing of it, but now it's just ridiculous