Here's four commands to demonstrate my problem:

My sample code:
$ cat prog.py 
import argparse
parse = argparse.ArgumentParser()
parse.add_argument("foo", nargs=2, metavar=("foo", "bar"))
args = parse.parse_args()
print(args)

This output could be friendlier:
$ python3 prog.py --help
Traceback (most recent call last):
  File "prog.py", line 4, in <module>
    args = parse.parse_args()
  File "/home/wena/cpython/Lib/argparse.py", line 1722, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/home/wena/cpython/Lib/argparse.py", line 1754, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/home/wena/cpython/Lib/argparse.py", line 1960, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/home/wena/cpython/Lib/argparse.py", line 1900, in consume_optional
    take_action(action, args, option_string)
  File "/home/wena/cpython/Lib/argparse.py", line 1828, in take_action
    action(self, namespace, argument_values, option_string)
  File "/home/wena/cpython/Lib/argparse.py", line 1015, in __call__
    parser.print_help()
  File "/home/wena/cpython/Lib/argparse.py", line 2347, in print_help
    self._print_message(self.format_help(), file)
  File "/home/wena/cpython/Lib/argparse.py", line 2314, in format_help
    formatter.add_arguments(action_group._group_actions)
  File "/home/wena/cpython/Lib/argparse.py", line 270, in add_arguments
    self.add_argument(action)
  File "/home/wena/cpython/Lib/argparse.py", line 255, in add_argument
    invocations = [get_invocation(action)]
  File "/home/wena/cpython/Lib/argparse.py", line 533, in _format_action_invocation
    metavar, = self._metavar_formatter(action, default)(1)
ValueError: too many values to unpack (expected 1)

On Python 3.2 (from Debian), I get "error: too few arguments". But with latest mercurial I get:
$ python3 prog.py
Traceback (most recent call last):
  File "prog.py", line 4, in <module>
    args = parse.parse_args()
  File "/home/wena/cpython/Lib/argparse.py", line 1722, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/home/wena/cpython/Lib/argparse.py", line 1754, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/home/wena/cpython/Lib/argparse.py", line 1973, in _parse_known_args
    ', '.join(required_actions))
TypeError: sequence item 0: expected str instance, tuple found

But this looks okay:
$ python3 prog.py a b
Namespace(foo=['a', 'b'])