Method and apparatus for managing item sequence numbers in an item processing system. The invention provides for overrun prevention and management of item sequence numbers, which form a part of an image key. In example embodiments, a buffer is provided to determine how close a current item sequence number (ISN) is permitted to be to an overrun value. When the current ISN reaches the buffer value, new entries of documents are prevented or at least restricted from being processed. In some embodiments, a “force mode” is provided in which a sorter can be made to start a new entry even if the buffer value has been exceeded. In such an embodiment, the system can be set up so that a hard stop is enforced when the ISN is within a certain range of the overrun value.