I've tested the docstring of the module subprocess.
Python 2.5 is OK, but Python 3.1 fails.

~ $ python2.5 -m doctest /usr/lib/python2.5/subprocess.py
...............
----------------------------------------------------------------------
Ran 15 tests in 0.012s

OK
~ $ python3 -m doctest /usr/lib/python3.1/subprocess.py
Traceback (most recent call last):
  File "/usr/lib/python3.1/runpy.py", line 128, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python3.1/runpy.py", line 34, in _run_code
    exec(code, run_globals)
  File "/usr/lib/python3.1/doctest.py", line 2637, in <module>
    sys.exit(_test())
  File "/usr/lib/python3.1/doctest.py", line 2626, in _test
    failures, _ = testmod(m)
  File "/usr/lib/python3.1/doctest.py", line 1847, in testmod
    for test in finder.find(m, name, globs=globs, extraglobs=extraglobs):
  File "/usr/lib/python3.1/doctest.py", line 854, in find
    self._find(tests, obj, name, module, source_lines, globs, {})
  File "/usr/lib/python3.1/doctest.py", line 908, in _find
    globs, seen)
  File "/usr/lib/python3.1/doctest.py", line 896, in _find
    test = self._get_test(obj, name, module, globs, source_lines)
  File "/usr/lib/python3.1/doctest.py", line 980, in _get_test
    filename, lineno)
  File "/usr/lib/python3.1/doctest.py", line 590, in get_doctest
    return DocTest(self.get_examples(string, name), globs,
  File "/usr/lib/python3.1/doctest.py", line 604, in get_examples
    return [x for x in self.parse(string, name)
  File "/usr/lib/python3.1/doctest.py", line 566, in parse
    self._parse_example(m, name, lineno)
  File "/usr/lib/python3.1/doctest.py", line 636, in _parse_example
    lineno + len(source_lines))
  File "/usr/lib/python3.1/doctest.py", line 722, in _check_prefix
    (lineno+i+1, name, line))
ValueError: line 11 of the docstring for subprocess.check_output has
inconsistent leading whitespace: "'"