diff --git a/addons/stock/stock.py b/addons/stock/stock.py
index dd041323033..1ac587bad46 100644
--- a/addons/stock/stock.py
+++ b/addons/stock/stock.py
@@ -1652,7 +1652,16 @@ class stock_move(osv.osv):
         'date_expected': fields.datetime('Scheduled Date', states={'done': [('readonly', True)]},required=True, select=True, help="Scheduled date for the processing of this move"),
         'product_id': fields.many2one('product.product', 'Product', required=True, select=True, domain=[('type','<>','service')],states={'done': [('readonly', True)]}),
 
-        'product_qty': fields.float('Quantity', digits_compute=dp.get_precision('Product Unit of Measure'), required=True,states={'done': [('readonly', True)]}),
+        'product_qty': fields.float('Quantity', digits_compute=dp.get_precision('Product Unit of Measure'),
+            required=True,states={'done': [('readonly', True)]},
+            help="This is the quantity of products from an inventory "
+                "point of view. For moves in the state 'done', this is the "
+                "quantity of products that were actually moved. For other "
+                "moves, this is the quantity of product that is planned to "
+                "be moved. Lowering this quantity does not generate a "
+                "backorder. Changing this quantity on assigned moves affects "
+                "the product reservation, and should be done with care."
+        ),
         'product_uom': fields.many2one('product.uom', 'Unit of Measure', required=True,states={'done': [('readonly', True)]}),
         'product_uos_qty': fields.float('Quantity (UOS)', digits_compute=dp.get_precision('Product Unit of Measure'), states={'done': [('readonly', True)]}),
         'product_uos': fields.many2one('product.uom', 'Product UOS', states={'done': [('readonly', True)]}),
@@ -1873,19 +1882,31 @@ class stock_move(osv.osv):
         result = {
                   'product_uos_qty': 0.00
           }
+        warning = {}
 
         if (not product_id) or (product_qty <=0.0):
             return {'value': result}
 
         product_obj = self.pool.get('product.product')
         uos_coeff = product_obj.read(cr, uid, product_id, ['uos_coeff'])
+        
+        # Warn if the quantity was decreased 
+        if ids:
+            for move in self.read(cr, uid, ids, ['product_qty']):
+                if product_qty < move['product_qty']:
+                    warning.update({
+                       'title': _('Information'),
+                       'message': _("By changing this quantity here, you accept the "
+                                "new quantity as complete: OpenERP will not "
+                                "automatically generate a back order.") })
+                break
 
         if product_uos and product_uom and (product_uom != product_uos):
             result['product_uos_qty'] = product_qty * uos_coeff['uos_coeff']
         else:
             result['product_uos_qty'] = product_qty
 
-        return {'value': result}
+        return {'value': result, 'warning': warning}
 
     def onchange_uos_quantity(self, cr, uid, ids, product_id, product_uos_qty,
                           product_uos, product_uom):
@@ -1899,19 +1920,29 @@ class stock_move(osv.osv):
         result = {
                   'product_qty': 0.00
           }
+        warning = {}
 
         if (not product_id) or (product_uos_qty <=0.0):
             return {'value': result}
 
         product_obj = self.pool.get('product.product')
         uos_coeff = product_obj.read(cr, uid, product_id, ['uos_coeff'])
+        
+        # Warn if the quantity was decreased 
+        for move in self.read(cr, uid, ids, ['product_uos_qty']):
+            if product_uos_qty < move['product_uos_qty']:
+                warning.update({
+                   'title': _('Warning: No Back Order'),
+                   'message': _("By changing the quantity here, you accept the "
+                                "new quantity as complete: OpenERP will not "
+                                "automatically generate a Back Order.") })
+                break
 
         if product_uos and product_uom and (product_uom != product_uos):
             result['product_qty'] = product_uos_qty / uos_coeff['uos_coeff']
         else:
             result['product_qty'] = product_uos_qty
-
-        return {'value': result}
+        return {'value': result, 'warning': warning}
 
     def onchange_product_id(self, cr, uid, ids, prod_id=False, loc_id=False,
                             loc_dest_id=False, partner_id=False):