Index: django/contrib/sessions/middleware.py
===================================================================
--- django/contrib/sessions/middleware.py	(revision 5559)
+++ django/contrib/sessions/middleware.py	(working copy)
@@ -37,6 +37,7 @@
         return self._session.get(key, default)
 
     def pop(self, key, *args):
+        self.modified = self.modified or key in self._session
         return self._session.pop(key, *args)
 
     def set_test_cookie(self):