Starting in 4.2.7, the following association mapping results in an error on flushing (regardless of whether there was a change to the collection).

<class name="User" table="`User`">
	<id name="name" length="32" />
	<map name="groups" table="UserGroup" cascade="all,delete-orphan" >
		<key column="name" />
		<map-key column="groupType" type="integer" />
		<many-to-many class="Group"  unique="true" >
			<column name="groupName"/>
		</many-to-many>
	</map>
</class>

Stacktrace:

org.hibernate.HibernateException: Unable to resolve property: null
	at org.hibernate.tuple.entity.EntityMetamodel.getPropertyIndex(EntityMetamodel.java:1215)
	at org.hibernate.persister.entity.AbstractEntityPersister.getPropertyIndex(AbstractEntityPersister.java:2406)
	at org.hibernate.engine.spi.EntityEntry.getLoadedValue(EntityEntry.java:289)
	at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:188)
	at org.hibernate.engine.internal.Cascade.cascadeCollectionElements(Cascade.java:381)
	at org.hibernate.engine.internal.Cascade.cascadeCollection(Cascade.java:321)
	at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:298)
	at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:161)
	at org.hibernate.engine.internal.Cascade.cascade(Cascade.java:118)
	at org.hibernate.event.internal.AbstractFlushingEventListener.cascadeOnFlush(AbstractFlushingEventListener.java:167)
	at org.hibernate.event.internal.AbstractFlushingEventListener.prepareEntityFlushes(AbstractFlushingEventListener.java:158)
	at org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:91)
	at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:55)
	at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1218)
	at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:421)
	at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.beforeTransactionCommit(JdbcTransaction.java:101)
	at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:177)
	at org.hibernate.test.manytomany.mapunique.ManyToManyUniqueDelOrphanTest.testManyToManyWithCascadeDeleteOrphan(ManyToManyUniqueDelOrphanTest.java:62)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
	at org.hibernate.testing.junit4.ExtendedFrameworkMethod.invokeExplosively(ExtendedFrameworkMethod.java:62)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
	at org.junit.internal.runners.statements.FailOnTimeout$StatementThread.run(FailOnTimeout.java:74)

I've attached a test case (org.hibernate.test.manytomany.mapunique.ManyToManyUniqueDelOrphanTest). The test case succeeds under 4.2.6, but fails under 4.2.7 (and master).

I believe this regression was caused by the fix for HHH-6484.