I ran into something very similar. In my case I have parent/children unidirectional one-to-many relation.

class Parent
{
    private ArrayList children;

    @OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @JoinColumn(name="parent_id", nullable = false)
    @OrderBy(value = "time")
    public List<Child> getChildred()
    {
        children.trimToSize();
        return Collections.synchronizedList(children);
    }

The test case that's failing is fairly standard:

Parent myParent = new Parent();
//some fields are set

Child child1 = new Child();
Child child2 = new Child();
//set child1 & child2 values through setters

ArrayList<Child> children = new ArrayList<Child>();
children.add(child1);
children.add(child2);
myParent.setChildren(children);

//Then essentially:

EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
em.merge(myParent);
em.flush();
em.getTransaction().commit();
em.close();

The stack trace is as follows:

java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
at org.hibernate.engine.internal.Cascade.cascadeCollectionElements(Cascade.java:418)
at org.hibernate.engine.internal.Cascade.cascadeCollection(Cascade.java:344)
at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:307)
at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:146)
at org.hibernate.engine.internal.Cascade.cascade(Cascade.java:95)
at org.hibernate.event.internal.AbstractSaveEventListener.cascadeAfterSave(AbstractSaveEventListener.java:456)
at org.hibernate.event.internal.DefaultMergeEventListener.entityIsTransient(DefaultMergeEventListener.java:239)
at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:173)
at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:69)
at org.hibernate.internal.SessionImpl.fireMerge(SessionImpl.java:839)
at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:821)
at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:826)
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.merge(AbstractEntityManagerImpl.java:1161)
--------------------------------- //here goes the code I posted above
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
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.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

*no* further _formatting_ is done here