The exception is the normal. In the current business world a major aspect of service quality is the ability to handle special customer requests like the possibility to change an order. I think a normal situation is that a customer sends an order to a supplie. 5 minutes or one day later he or she wants to change the order. The colour of the ordered shirt should be green instead of blue and the customer wants to add an additional yellow shirt. The reaction can be different depending on the status of the process execution of the order handling process . Sometimes it is possible to accept the change request, and sometimes it is not, e.g., when the change request arrives after shipment was already initiated.
Try to describe such a process as an Event-driven Process Chain (EPC) or in BPMN. Up to now, I could not find a transparent and reasonable solution. If you find one let me know.