1) I create Hibernate's object using:
MyObject object = new MyObject(...);
2)Then i save it in HttpSession attribute:
session.addAttribute("object", object);
3) This object transfered from servlet to servlet, sometimes, the fields of this object becomes changed. At the ending, i do in one of my servlets:
Object obj = session.getAttribute("object");
Session sess = HibernateUtil.currentSession();
MyObject object = (MyObject)obj;
sess.save(object);
sess.flush();
4) I repeat steps 1-3 several times, but on 3rd or 4th try i receive an exception:
objectnet.sf.hibernate.HibernateException: Illegal attempt to associate a collection with two open sessions

I'm using ONLY one session object at one time ! How it could happens ???

Ooooooooops, i forgot to invoke closeSession() method...i think that's the reason....let's see...

Shit ! What a stupid mistake !

Solved...