Index: Lib/urllib2.py
===================================================================
--- Lib/urllib2.py	(revision 78138)
+++ Lib/urllib2.py	(working copy)
@@ -390,6 +390,12 @@
 
         response = self._open(req, data)
 
+        # remove auth headers that were sent in a previous request
+        for key in [HTTPBasicAuthHandler.auth_header,
+                    ProxyBasicAuthHandler.auth_header]:
+            if key in req.headers:
+                req.headers.pop(key)
+
         # post-process response
         meth_name = protocol+"_response"
         for processor in self.process_response.get(protocol, []):