STAN does not automatically check if the sum of the masses of substances in a flow is larger than the mass of goods. Might be a good idea to implement a warning with respect to this problem. I put it on our to do list.
If you want the sum of the masses of the defined substances in a flow to be equal to the mass of goods, tick "Use subgood layers" under "Extras > Options > Tab Common > Region Subgoods".
