diff -r 9c8d31d69044 Lib/mimetypes.py
--- a/Lib/mimetypes.py	Mon Jan 06 11:10:08 2014 -0800
+++ b/Lib/mimetypes.py	Mon Apr 14 08:54:34 2014 -0700
@@ -171,6 +171,7 @@
             for ext in self.types_map_inv[False].get(type, []):
                 if ext not in extensions:
                     extensions.append(ext)
+        extensions.sort()
         return extensions
 
     def guess_extension(self, type, strict=True):
diff -r 9c8d31d69044 Lib/test/test_mimetypes.py
--- a/Lib/test/test_mimetypes.py	Mon Jan 06 11:10:08 2014 -0800
+++ b/Lib/test/test_mimetypes.py	Mon Apr 14 08:54:34 2014 -0700
@@ -6,16 +6,27 @@
 
 from test import support
 
-# Tell it we don't know about external files:
-mimetypes.knownfiles = []
-mimetypes.inited = False
-mimetypes._default_mime_types()
+mimetypes_globals = ('suffix_map', 'types_map', 'encodings_map',
+                     'common_types', 'inited', '_db', 'knownfiles')
 
 
 class MimeTypesTestCase(unittest.TestCase):
     def setUp(self):
+        # Save mimetypes global module state
+        self.saved = {}
+        for mod_var in mimetypes_globals:
+            self.saved[mod_var] = getattr(mimetypes, mod_var)
+        # Tell it we don't know about external files:
+        mimetypes.knownfiles = []
+        mimetypes.inited = False
+        mimetypes._default_mime_types()
+
         self.db = mimetypes.MimeTypes()
 
+    def tearDown(self):
+        for mod_var in mimetypes_globals:
+            setattr(mimetypes, mod_var, self.saved[mod_var])
+
     def test_default_data(self):
         eq = self.assertEqual
         eq(self.db.guess_type("foo.html"), ("text/html", None))
@@ -56,7 +67,7 @@
         # test_urllib2 is run before test_mimetypes, global state is modified
         # such that the 'all' set will have more items in it.
         all = set(self.db.guess_all_extensions('text/plain', strict=True))
-        unless(all >= set(['.bat', '.c', '.h', '.ksh', '.pl', '.txt']))
+        unless(len(all) >= len(set(['.bat', '.c', '.h', '.ksh', '.pl', '.txt'])))
         # And now non-strict
         all = self.db.guess_all_extensions('image/jpg', strict=False)
         all.sort()
@@ -78,6 +89,38 @@
         self.assertEqual(exts, ['.g3', '.g\xb3'])
 
 
+class MimeTypesGlobalsTestCase(unittest.TestCase):
+    def setUp(self):
+        self.db = mimetypes.MimeTypes()
+        self.saved = {}
+        for mod_var in mimetypes_globals:
+            self.saved[mod_var] = getattr(mimetypes, mod_var)
+
+    def tearDown(self):
+        for mod_var in mimetypes_globals:
+            setattr(mimetypes, mod_var, self.saved[mod_var])
+
+    def test_extensions_list_stable(self):
+        # Test that the sort of extensions is stable
+        # http://bugs.python.org/issue4963
+        eq = self.assertEqual
+
+        # Find a suitable mime type for tests (needs more than one suffix)
+        for test_type in mimetypes._db.types_map_inv[True]:
+            exts = mimetypes.guess_all_extensions(test_type)
+            if len(exts) > 1:
+                break
+
+        results = set()
+        for i in range(0, 20):
+            # Note -- this isn't guaranteed to catch that extension order is
+            # unstable.  It's just statistically likely.
+            results.add(tuple(mimetypes._db.guess_all_extensions(
+                    test_type, strict=True)))
+            mimetypes.init()
+        eq(len(results), 1)
+
+
 @unittest.skipUnless(sys.platform.startswith("win"), "Windows only")
 class Win32MimeTypesTestCase(unittest.TestCase):
     def setUp(self):
@@ -103,6 +146,7 @@
 
 def test_main():
     support.run_unittest(MimeTypesTestCase,
+        MimeTypesGlobalsTestCase,
         Win32MimeTypesTestCase
         )