The same happens to me ...

=== modified file 'point_of_sale/point_of_sale.py' --- point_of_sale/point_of_sale.py 2012-12-06 14:56:32 +0000 +++ point_of_sale/point_of_sale.py 2013-01-17 12:08:40 +0000 @@ -290,7 +290,7 @@

  _constraints = [
         (_check_unicity, "You cannot create two active sessions with the same responsible!", ['user_id', 'state']),
-        (_check_pos_config, "You cannot create two active sessions related to the same point of sale!", ['config_id']),
+        (_check_pos_config, "You cannot create two active sessions related to the same point of sale! Use not used point of sale", ['config_id']),
     ]

 def create(self, cr, uid, values, context=None):

=== modified file 'point_of_sale/wizard/pos_session_opening.py' --- point_of_sale/wizard/pos_session_opening.py 2012-12-06 14:56:32 +0000 +++ point_of_sale/wizard/pos_session_opening.py 2013-01-17 12:08:40 +0000 @@ -88,6 +88,7 @@

    session_ids = proxy.search(cr, uid, [
             ('state', '!=', 'closed'),
             ('config_id', '=', config_id),
+            ('user_id', '=', uid),
         ], context=context)
         if session_ids:
             session = proxy.browse(cr, uid, session_ids[0], context=context)

apply this patch in POS

also never try to start a new session without closing a previous session. This is a bug in POS that it cannot handle two sessions simultaneously. The above patch does not resolve the bug but prevents the system from breaking at any stage inside POS.