The json decoder doesn't pass tests when the C optimizations in the _json module aren't available.  The actual test failures appear fairly superficial, but aren't the limit of the problems.

Empty objects are converted to empty lists instead of empty dictionaries when _json isn't available; test attached.

Issue 5723 notes that the pure-Python implementation isn't tested; fixing that might have avoided these problems.

Test failures from Python 2.6.5:

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ERROR: test_c_encode_basestring_ascii (json.tests.test_encode_basestring_ascii.TestEncodeBaseStringAscii)
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Traceback (most recent call last):
  File "/home/fdrake/src/Python-2.6.5/Lib/json/tests/test_encode_basestring_ascii.py", line 29, in test_c_encode_basestring_ascii
    self._test_encode_basestring_ascii(json.encoder.c_encode_basestring_ascii)
  File "/home/fdrake/src/Python-2.6.5/Lib/json/tests/test_encode_basestring_ascii.py", line 32, in _test_encode_basestring_ascii
    fname = encode_basestring_ascii.__name__
AttributeError: 'NoneType' object has no attribute '__name__'

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ERROR: test_c_scanstring (json.tests.test_scanstring.TestScanString)
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Traceback (most recent call last):
  File "/home/fdrake/src/Python-2.6.5/Lib/json/tests/test_scanstring.py", line 13, in test_c_scanstring
    self._test_scanstring(json.decoder.c_scanstring)
  File "/home/fdrake/src/Python-2.6.5/Lib/json/tests/test_scanstring.py", line 17, in _test_scanstring
    scanstring('"z\\ud834\\udd20x"', 1, None, True),
TypeError: 'NoneType' object is not callable

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FAIL: test_encode_basestring_ascii (json.tests.test_speedups.TestSpeedups)
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Traceback (most recent call last):
  File "/home/fdrake/src/Python-2.6.5/Lib/json/tests/test_speedups.py", line 13, in test_encode_basestring_ascii
    self.assertEquals(encoder.encode_basestring_ascii.__module__, "_json")
AssertionError: 'json.encoder' != '_json'

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FAIL: test_scanstring (json.tests.test_speedups.TestSpeedups)
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Traceback (most recent call last):
  File "/home/fdrake/src/Python-2.6.5/Lib/json/tests/test_speedups.py", line 9, in test_scanstring
    self.assertEquals(decoder.scanstring.__module__, "_json")
AssertionError: 'json.decoder' != '_json'


Test failures from 2.7:

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ERROR: test_c_scanstring (json.tests.test_scanstring.TestScanString)
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Traceback (most recent call last):
  File "/home/fdrake/src/Python-2.7/Lib/json/tests/test_scanstring.py", line 13, in test_c_scanstring
    self._test_scanstring(json.decoder.c_scanstring)
  File "/home/fdrake/src/Python-2.7/Lib/json/tests/test_scanstring.py", line 17, in _test_scanstring
    scanstring('"z\\ud834\\udd20x"', 1, None, True),
TypeError: 'NoneType' object is not callable

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FAIL: test_encode_basestring_ascii (json.tests.test_speedups.TestSpeedups)
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Traceback (most recent call last):
  File "/home/fdrake/src/Python-2.7/Lib/json/tests/test_speedups.py", line 12, in test_encode_basestring_ascii
    self.assertEquals(encoder.encode_basestring_ascii.__module__, "_json")
AssertionError: 'json.encoder' != '_json'

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FAIL: test_scanstring (json.tests.test_speedups.TestSpeedups)
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Traceback (most recent call last):
  File "/home/fdrake/src/Python-2.7/Lib/json/tests/test_speedups.py", line 8, in test_scanstring
    self.assertEquals(decoder.scanstring.__module__, "_json")
AssertionError: 'json.decoder' != '_json'