diff -r a88310d86455 Lib/argparse.py
--- a/Lib/argparse.py	Sun Apr 14 19:22:47 2013 +0200
+++ b/Lib/argparse.py	Wed Apr 17 15:06:13 2013 -0700
@@ -1856,8 +1856,13 @@
                             action = optionals_map[option_string]
                             explicit_arg = new_explicit_arg
                         else:
-                            msg = _('ignored explicit argument %r')
-                            raise ArgumentError(action, msg % explicit_arg)
+                            if False:
+                                msg = _('ignored explicit argument %r')
+                                raise ArgumentError(action, msg % explicit_arg)
+                            else:
+                                # alt handling of unknown explicit_arg
+                                extras.append(explicit_arg)
+                                return start_index + 1
 
                     # if the action expect exactly one argument, we've
                     # successfully matched the option; exit the loop