diff -r 44f455e6163d Lib/argparse.py
--- a/Lib/argparse.py	Thu Jun 27 12:23:29 2013 +0200
+++ b/Lib/argparse.py	Fri Jul 25 14:25:59 2014 -0700
@@ -1544,6 +1544,10 @@
         super(_ArgumentGroup, self)._remove_action(action)
         self._group_actions.remove(action)
 
+    def add_argument_group(self, *args, **kwargs):
+        raise ValueError('Cannot nest groups')
+        # or NotImplementedError
+
 
 class _MutuallyExclusiveGroup(_ArgumentGroup):
 
@@ -1564,6 +1568,10 @@
         self._container._remove_action(action)
         self._group_actions.remove(action)
 
+    def add_mutually_exclusive_group(self, **kwargs):
+        raise ValueError('Cannot nest mutually exclusive groups')
+        # or NotImplementedError
+
 
 class ArgumentParser(_AttributeHolder, _ActionsContainer):
     """Object for parsing command line strings into Python objects.