Dan, form your description it is the Combo's Control Source that is the problem, "The Control Source for the Combo was Orders_Table.[Order#] field".
Are you sure that the Field is "Updateable", you haven't made it an Autonumber by mistake, Also is the actual Recordset for the Form Updateable?
Is it a Query, can you add an order # in it manually?
Also in the Form manually?