What is a IncompatibleClassChangeError exception in Java?

i am working on a small application and I am trying to use Hibernate Annotations to map my entities. I wanted to test if everything is alright when i got this exception :

    Exception in thread "main" java.lang.ExceptionInInitializerError
 at fr.cc2i.intervention.dao.main.Main$HibernateUtil.<clinit>(Main.java:48)
 at fr.cc2i.intervention.dao.main.Main.test(Main.java:21)
 at fr.cc2i.intervention.dao.main.Main.main(Main.java:32)
Caused by: java.lang.IncompatibleClassChangeError: Implementing class
 at java.lang.ClassLoader.defineClass1(Native Method)
 at java.lang.ClassLoader.defineClassCond(ClassLoader.java:632)
 at java.lang.ClassLoader.defineClass(ClassLoader.java:616)
 at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
 at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
 at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
 at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
 at java.security.AccessController.doPrivileged(Native Method)
 at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
 at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
 at fr.cc2i.intervention.dao.main.Main$HibernateUtil.<clinit>(Main.java:44)
 ... 2 more

Can someone explain what is this exception ? This is the first time i see it. Here is the main of my application :

 package fr.cc2i.intervention.dao.main;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;

import fr.cc2i.intervention.dao.beans.Client;
import fr.cc2i.intervention.dao.beans.Contrat;

public class Main {

 public static void test(){
  Client c = new Client();
  c.setCode("123343");
  c.setAdresse("fkhdhdmh");
  c.setNom("dgsfhgsdfgs");
  c.setPhone("53456464");
  c.setContrat(new Contrat());

  Session session = HibernateUtil.getSession();
        session.beginTransaction();        
        session.save(c);
        session.getTransaction().commit();

 }

 /**
  * @param args
  */
 public static void main(String[] args) {
  Main.test();

 }

 public static class HibernateUtil {

 private static final SessionFactory sessionFactory;
     static {
         try {
             sessionFactory = new AnnotationConfiguration()
                     .configure().buildSessionFactory();
         } catch (Throwable ex) {
             // Log exception!
             throw new ExceptionInInitializerError(ex);
         }
     }

     public static Session getSession()
             throws HibernateException {
         return sessionFactory.openSession();
     }
 }

}

My hibernate config is very simple :

    <!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
 <session-factory>
  <!-- Database connection settings -->
  <property name="connection.driver_class">org.hsqldb.jdbcDriver</property>
  <property name="connection.url">jdbc:hsqldb:hsql://localhost</property>
  <property name="connection.username">sa</property>
  <property name="connection.password"></property>

  <!-- JDBC connection pool (use the built-in) -->
  <property name="connection.pool_size">1</property>

  <!-- SQL dialect -->
  <property name="dialect">org.hibernate.dialect.HSQLDialect</property>

  <!-- Enable Hibernate's automatic session context management -->
  <property name="current_session_context_class">thread</property>

  <!-- Disable the second-level cache  -->
  <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

  <!-- Echo all executed SQL to stdout -->
  <property name="show_sql">true</property>

  <!-- Drop and re-create the database schema on startup -->
  <property name="hbm2ddl.auto">update</property>

  <mapping package="fr.cc2i.intervention.dao.beans.Client" />
  <mapping class="fr.cc2i.intervention.dao.beans.Contrat" />
  <mapping class="fr.cc2i.intervention.dao.beans.Intervention" />
  <mapping class="fr.cc2i.intervention.dao.beans.Technicien" />



 </session-factory>
</hibernate-configuration>

Here is the hibernate maven dependency i am using :

<properties>
        <org.springframework.version>3.0.3.RELEASE</org.springframework.version>
        <hibernate.version>3.6.0.Beta1</hibernate.version></properties>

    <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-entitymanager</artifactId>
            <version>${hibernate.version}</version>
            <type>jar</type>
            <scope>compile</scope>
        </dependency>

Can someone help me please ??