@Entity
public class MyEntity {
  @Id
  @GeneratedValue
  public Integer id;
}

MyEntity myEntity = new MyEntity();
entityManager.persist(myEntity);
entityManager.flush();
entityManager.remove(myEntity);
entityManager.flush();
entityManager.persist(myEntity);

org.hibernate.PersistentObjectException: detached entity passed to persist: org.hibernate.bugs.JPAUnitTestCase$MyEntity