diff -r da10196b94f4 Lib/test/test_filecmp.py
--- a/Lib/test/test_filecmp.py	Sun Nov 17 13:15:51 2013 +0000
+++ b/Lib/test/test_filecmp.py	Sat Nov 23 11:36:05 2013 -0300
@@ -7,6 +7,7 @@
         self.name = support.TESTFN
         self.name_same = support.TESTFN + '-same'
         self.name_diff = support.TESTFN + '-diff'
+        self.name_diff_same_size = support.TESTFN + '-diff-same-size'
         data = 'Contents of file go here.\n'
         for name in [self.name, self.name_same, self.name_diff]:
             output = open(name, 'w')
@@ -16,12 +17,16 @@
         output = open(self.name_diff, 'a+')
         output.write('An extra line.\n')
         output.close()
+        output = open(self.name_diff_same_size, 'w')
+        output.write('X' + data[1:])
+        output.close()
         self.dir = tempfile.gettempdir()
 
     def tearDown(self):
         os.unlink(self.name)
         os.unlink(self.name_same)
         os.unlink(self.name_diff)
+        os.unlink(self.name_diff_same_size);
 
     def test_matching(self):
         self.assertTrue(filecmp.cmp(self.name, self.name_same),
@@ -38,6 +43,10 @@
                     "Mismatched files compare as equal")
         self.assertFalse(filecmp.cmp(self.name, self.dir),
                     "File and directory compare as equal")
+        self.assertFalse(filecmp.cmp(self.name,
+                            self.name_diff_same_size,
+                            shallow=False),
+                         "Mismatched files with same size compare as equal")
 
     def test_cache_clear(self):
         first_compare = filecmp.cmp(self.name, self.name_same, shallow=False)