diff --git a/django/templatetags/i18n.py b/django/templatetags/i18n.py
index c7202e2..cd7a3a7 100644
--- a/django/templatetags/i18n.py
+++ b/django/templatetags/i18n.py
@@ -122,7 +122,12 @@ class BlockTranslateNode(Node):
         result = re.sub(u'%(?!\()', u'%%', result)
         data = dict([(v, _render_value_in_context(context.get(v, ''), context)) for v in vars])
         context.pop()
-        return result % data
+        try:
+            result = result % data
+        except KeyError:
+            with translation.override(None):
+                result = self.render(context)
+        return result
 
 
 class LanguageNode(Node):
diff --git a/django/utils/translation/__init__.py b/django/utils/translation/__init__.py
index acf4d8d..8ad9c5d 100644
--- a/django/utils/translation/__init__.py
+++ b/django/utils/translation/__init__.py
@@ -115,7 +115,10 @@ class override(object):
         self.old_language = get_language()
 
     def __enter__(self):
-        activate(self.language)
+        if self.language is not None:
+            activate(self.language)
+        else:
+            deactivate_all()
 
     def __exit__(self, exc_type, exc_value, traceback):
         if self.deactivate:
diff --git a/docs/ref/utils.txt b/docs/ref/utils.txt
index 0849a21..299d026 100644
--- a/docs/ref/utils.txt
+++ b/docs/ref/utils.txt
@@ -537,15 +537,19 @@ For a complete discussion on the usage of the following see the
     useful when we want delayed translations to appear as the original string
     for some reason.
 
-.. function:: override(language)
+.. function:: override(language, deactivate=False)
 
     .. versionadded:: 1.4
 
     A Python context manager that uses
     :func:`django.utils.translation.activate` to fetch the translation object
     for a given language, installing it as the translation object for the
-    current thread and deinstalls it again on exit with
-    :func:`django.utils.translation.deactivate`.
+    current thread and reinstall the previous active language on exit.
+    Optionally it can simply deinstall the temporary translation on exit with
+    :func:`django.utils.translation.deactivate` if the deactivate argument is
+    True.
+    If you pass None as the language argument, a NullTranslations() instance is
+    installed while the context is active.
 
 .. function:: get_language()
 
diff --git a/tests/regressiontests/i18n/other/locale/fr/LC_MESSAGES/django.mo b/tests/regressiontests/i18n/other/locale/fr/LC_MESSAGES/django.mo
new file mode 100644
index 0000000..f0a2179
Binary files /dev/null and b/tests/regressiontests/i18n/other/locale/fr/LC_MESSAGES/django.mo differ
diff --git a/tests/regressiontests/i18n/other/locale/fr/LC_MESSAGES/django.po b/tests/regressiontests/i18n/other/locale/fr/LC_MESSAGES/django.po
new file mode 100644
index 0000000..8e9b9e1
--- /dev/null
+++ b/tests/regressiontests/i18n/other/locale/fr/LC_MESSAGES/django.po
@@ -0,0 +1,21 @@
+# SOME DESCRIPTIVE TITLE.
+# Copyright (C) YEAR THE PACKAGE'S COPYRIGHT HOLDER
+# This file is distributed under the same license as the PACKAGE package.
+# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
+#
+msgid ""
+msgstr ""
+"Project-Id-Version: django tests\n"
+"Report-Msgid-Bugs-To: \n"
+"POT-Creation-Date: 2010-02-14 17:33+0100\n"
+"PO-Revision-Date: 2011-01-21 21:37-0300\n"
+"Last-Translator: Claude\n"
+"Language-Team: fr <fr@li.org>\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=UTF-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Plural-Forms: nplurals=2; plural=(n != 1)\n"
+
+#: template.html:3
+msgid "My name is %(person)s."
+msgstr "Mon nom est %(personne)s."
diff --git a/tests/regressiontests/i18n/tests.py b/tests/regressiontests/i18n/tests.py
index 0b955ad..e5a08da 100644
--- a/tests/regressiontests/i18n/tests.py
+++ b/tests/regressiontests/i18n/tests.py
@@ -9,6 +9,7 @@ from threading import local
 from django.conf import settings
 from django.template import Template, Context
 from django.test import TestCase, RequestFactory
+from django.test.utils import override_settings
 from django.utils.formats import (get_format, date_format, time_format,
     localize, localize_input, iter_format_modules, get_format_modules)
 from django.utils.importlib import import_module
@@ -34,6 +35,9 @@ class TranslationTests(TestCase):
         with translation.override('pl'):
             self.assertEqual(get_language(), 'pl')
         self.assertEqual(get_language(), 'de')
+        with translation.override(None):
+            self.assertEqual(get_language(), settings.LANGUAGE_CODE)
+        self.assertEqual(get_language(), 'de')
         deactivate()
 
     def test_lazy_objects(self):
@@ -129,6 +133,23 @@ class TranslationTests(TestCase):
         self.assertEqual(to_language('en_US'), 'en-us')
         self.assertEqual(to_language('sr_Lat'), 'sr-lat')
 
+    @override_settings(
+        LOCALE_PATHS = (
+            os.path.join(os.path.dirname(os.path.abspath(__file__)), 'other', 'locale'),
+        ),
+    )
+    def test_bad_placeholder(self):
+        """
+        Error in translation file should not crash template rendering
+        (%(person)s is translated as %(personne)s in fr.po)
+        """
+        from django.template import Template, Context
+        with translation.override('fr'):
+            t = Template('{% load i18n %}{% blocktrans %}My name is {{ person }}.{% endblocktrans %}')
+            c = Context({'person': u"James"})
+            rendered = t.render(c)
+            self.assertEqual(rendered, u"My name is James.")
+
 
 class FormattingTests(TestCase):
 
