diff -r 5163fb8dc61f Lib/test/_test_multiprocessing.py
--- a/Lib/test/_test_multiprocessing.py	Tue Mar 10 14:47:41 2015 +0200
+++ b/Lib/test/_test_multiprocessing.py	Tue Mar 10 12:18:17 2015 -0500
@@ -423,6 +423,19 @@
         self.parent_conn.close()
         self.child_conn.close()
 
+def is_error_on_warning_set():
+    """Returns True when tests are run with -Werror, False otherwise.  When
+    the equivalent of 'warnings.simplefilter("error", Warning)' has been set,
+    any warnings will trigger output sent to stderr but some of the tests
+    in this module examine what appears on stderr when exiting a Process.
+    This utility helps such tests adjust accordingly.
+    """
+    import warnings
+    gen_action_category = ((action, category) \
+        for action, _pattern, category, *_extras in warnings.filters)
+    error_on_warning = ("error", Warning) in gen_action_category
+    return error_on_warning
+
 class _TestSubclassingProcess(BaseTestCase):
 
     ALLOWED_TYPES = ('processes',)
@@ -480,7 +493,12 @@
             self.assertEqual(p.exitcode, code)
 
             with open(testfn, 'r') as f:
-                self.assertEqual(f.read().rstrip(), str(reason))
+                if is_error_on_warning_set():
+                    # Ignore any spurious warnings (via -Werror) sent to stderr
+                    contents = f.read().splitlines()
+                    self.assertEqual(contents[0], str(reason))
+                else:
+                    self.assertEqual(f.read().rstrip(), str(reason))
 
         for reason in (True, False, 8):
             p = self.Process(target=sys.exit, args=(reason,))