For every message shown in a sequence or collaboration diagram, verify that an operation on the sending class is responsible for sending the event and an operation on the receiving class expects the event and handles it. Verify that there is an association or aggregation on the class diagram between the sending and receiving classes. Add the relationship to the class diagram if it is missing. Finally, if a state transition diagram for the class exists, verify that the event is represented on the diagram for the receiving class. This is needed because the diagram shows all the events that a class may receive.