Assuming I branch of veto to stop the insertion process, Hibernate throws the following, which is technically correct. At least this would come back as a java.lang.IllegalStateException that you could perhaps catch in Grails. When I speak with the team, I'll see if anything else can be done, but we do have to work without the expectations of JPA.

Caused by: org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: org.hibernate.event.EventEvictionTest$B