Civilian Shipping Contracts are overfilling.
After a civilian ship loads an installation at the supply location, the "Assigned" amount of the demand contract reduces by 1.
If a new supply contract for this installation is created before this ship finishes unloading at the destination, the civvies will use that supply to fulfill the original demand contract again.
This can happen over and over for a single demand contract.
A civ ship will eventually deliver the installation, causing that demand contract to be deleted. But all the others en route will finish their delivery.
Steps to reproduce:
0) Ensure that a shipping company exists and that no supply or demand contracts currently exist.
1) Create a supply contract for 1 installation at Colony A (make sure Colony A has at least 1 such installation).
2) Create a demand contract for 1 of the same installation at Colony B (must be in same system as Colony A, or a jumpgate route must exist from A to B).
3) Advance time until a civilian freighter has been assigned to deliver the installation. Note that the "Assigned" amount has changed to 1 for both the supply and the demand contract.
4) Advance time until the civilian freighter has finished loading the installation at Colony A and begun moving to B. Note that the supply contract has disappeared, and the "Assigned" amount
has changed back to 0 for the demand contract.
5) Create a new supply contract for 1 of the same installation at Colony A. Do not create any matching demand contract.
6) Advance time. If/when a civilian freighter becomes available before the original freighter has unloaded the installation at B, the newly available freighter will be assigned to deliver the installation from A to B. The supply and demand contract "Assigned" amounts will change to 1 and the ship will move to A, load the installation (which reduces the "Assigned" amount on the demand contract back to 0), and begin moving to B.
7) If you like, go back to step 5 and do it again. See how many civvies you can get loaded up before the original freighter finally delivers the installation.
Fixed for v1.14: http://aurora2.pentarch.org/index.php?topic=12523.msg151550#msg151550