`nan != 'KORD'` error in test_multiple_date_cols_chunked 
Running pandas tests as part of scipy-stack in multiple OSX / Python install variations:

https://travis-ci.org/matthew-brett/scipy-stack-osx-testing/builds/26419147

For at least Python.org 3.3.5, no virtualenv, I see this error:

```
======================================================================
FAIL: test_multiple_date_cols_chunked (pandas.io.tests.test_parsers.TestCParserLowMemory)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/site-packages/pandas/io/tests/test_parsers.py", line 1740, in test_multiple_date_cols_chunked
    tm.assert_frame_equal(chunks[2], df[4:])
File "/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/site-packages/pandas/util/testing.py", line 585, in assert_frame_equal
    check_exact=check_exact)
File "/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/site-packages/pandas/util/testing.py", line 532, in assert_series_equal
    assert_almost_equal(left.values, right.values, check_less_precise)
File "testing.pyx", line 58, in pandas._testing.assert_almost_equal (pandas/src/testing.c:2536)
File "testing.pyx", line 93, in pandas._testing.assert_almost_equal (pandas/src/testing.c:1859)
File "testing.pyx", line 69, in pandas._testing.assert_almost_equal (pandas/src/testing.c:1556)
AssertionError: nan != 'KORD'
```

See: https://travis-ci.org/matthew-brett/scipy-stack-osx-testing/jobs/26419150

For the exact same set of packages / Python version but in a virtualenv, I don't see the error:

https://travis-ci.org/matthew-brett/scipy-stack-osx-testing/jobs/26419153

Is there something random in the test or the code that could explain this?
