Inconsistency in some calculations in pd.DataFrames with large number of rows
Strange behaviour that appears only with a large number of rows.

``` python
import pandas as pd
import numpy as np

np.random.seed(0)
data = pd.DataFrame(np.random.rand(500000, 4))

def sum_products(data):
    data_np = sum(data.values[:,0] * data.values[:,1] * data.values[:,2] * data.values[:,3])
    data_pd = sum(data.ix[:,0] * data.ix[:,1] * data.ix[:,2] * data.ix[:,3])

    return [data_np, data_pd]

calls = []
for _ in range(25):
    test = sum_products(data)
    calls.append(test)

print('[Numpy, Pandas]', calls )

Out[6]:
[Numpy, Pandas]
[[31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 0.0],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 0.0],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 0.0],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517],
 [31234.084575876517, 31234.084575876517]]
```

This bug appears randomly, but using this combination (4 columns, 500k rows and 25 repetitions) I can see it at least once almost all the executions.

I have tried several `Pandas` versions (0.13-0.17) with `Python 3.4` and the bug is still there.
HOWEVER, using `Python 2.7` it works prefectly (at least after several executions).

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Windows
OS-release: 8
machine: AMD64
processor: Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: es_ES

pandas: 0.17.0
nose: 1.3.7
pip: 7.1.2
setuptools: 18.4
Cython: 0.23.3
numpy: 1.10.1
scipy: 0.16.0
statsmodels: None
IPython: 4.0.0
sphinx: 1.2.3
patsy: 0.4.0
dateutil: 2.4.1
pytz: 2015.6
blosc: None
bottleneck: None
tables: None
numexpr: 2.4.4
matplotlib: 1.4.3
openpyxl: 2.0.2
xlrd: 0.9.4
xlwt: None
xlsxwriter: 0.7.5
lxml: 3.4.4
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.8
pymysql: None
psycopg2: None
```
