If all item slots except 6 are full, and you try to move an item on the quickbar into slot 6, it refuses to work. For some reason, however, slot 7 is the only exception to this rule. But only when there is at least one other open space on the bar. If all quickbar slots are full, swapping items on the quickbar with Slot 6 is impossible (including from slot 7). In most other cases, such as the bar being completely empty or 2 or more slots being open, if you try to put an item in slot 6 it will force the item back into your inventory or send it to the lowest available open slot on the bar. It seems arbitrary as to which method it chooses. Conclusion: slot 6 does not want to be a team player and should be sent to time-out.
