This happens to me in two instances, both occuring when I ship more than is available in iciloc and iciqty.
1. When I ship more than his on hand and take the overship inventory from a different location, a new line is created.
2. When I ship more than on hand and do not take the inventory from a different location, a new line is created with the serial number _overship. This also creates a new record in iciqty with the serial number _overship.
Hope this helps
Glen