Caused by: java.util.ConcurrentModificationException
	at java.util.LinkedList$ListItr.checkForComodification(LinkedList.java:966)
	at java.util.LinkedList$ListItr.next(LinkedList.java:888)

<plugin>
  <groupId>org.hibernate.orm.tooling</groupId>
  <artifactId>hibernate-enhance-maven-plugin</artifactId>
  <version>5.3.6.Final</version>
  <executions>
	<execution>
	  <goals>
		<goal>enhance</goal>
	  </goals>
	  <configuration>
		<failOnError>true</failOnError>
		<enableLazyInitialization>true</enableLazyInitialization>
		<enableDirtyTracking>true</enableDirtyTracking>
		<enableAssociationManagement>true</enableAssociationManagement>
	  </configuration>
	</execution>
  </executions>
</plugin>

import java.util.ArrayList;
import java.util.List;

import javax.inject.Inject;
import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.transaction.UserTransaction;

import org.jboss.arquillian.junit.Arquillian;
import org.junit.Test;
import org.junit.runner.RunWith;


@RunWith(Arquillian.class)
public class HibernateMavenBytecodeEnhancedAssociationTest {

    @Inject
    private EntityManager em;

    @Inject
    private UserTransaction utx;

    @Test
    public void testAssociation() throws Exception {

        Long parentId;

        {
            utx.begin();
            em.joinTransaction();

            Parent parent = new Parent();
            em.persist(parent);
            parentId = parent.getId();

            List<Child> children = new ArrayList<>();
            for (long i = 0; i < 10; i++) {
                Child child = new Child(parent);
                children.add(child);
            }

            parent.setChildren(children);

            utx.commit();
        }

        {
            utx.begin();
            em.joinTransaction();

            Parent parent = em.find(Parent.class, parentId);

            List<Child> newChildren = new ArrayList<>();
            for (long i = 0; i < 10; i++) {
                Child child = new Child(parent);
                newChildren.add(child);
            }

            parent.setChildren(newChildren);

            utx.commit();
        }
    }

    @Entity
    public static class Parent {

        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        private Long id;

        @OneToMany(mappedBy = "parent", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
        private List<Child> children = new ArrayList<>();

        public Long getId() {
            return id;
        }

        public void setId(Long id) {
            this.id = id;
        }

        public List<Child> getChildren() {
            return children;
        }

        public void setChildren(List<Child> children) {
            this.children = children;
        }

        public void addChild(Child child) {
            children.add(child);
        }

        @Override
        public String toString() {
            return "Parent{" +
                    "id=" + id +
                    '}';
        }
    }

    @Entity
    public static class Child {

        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        private Long id;

        @ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
        @JoinColumn(name = "parent_id")
        private Parent parent;

        public Child() {
        }

        public Child(Parent parent) {
            this.parent = parent;
        }

        public Long getId() {
            return id;
        }

        public void setId(Long id) {
            this.id = id;
        }

        public Parent getParent() {
            return parent;
        }

        public void setParent(Parent parent) {
            this.parent = parent;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;

            Child child = (Child) o;

            if (id != null ? !id.equals(child.id) : child.id != null) return false;
            return parent != null ? parent.equals(child.parent) : child.parent == null;
        }

        @Override
        public int hashCode() {
            int result = id != null ? id.hashCode() : 0;
            result = 31 * result + (parent != null ? parent.hashCode() : 0);
            return result;
        }
    }
}