Subtraction causes intermittent error
I came across an intermittent issue that took some experimentation to track down. I was using a oneliner function for taring dataframes (or series, I've noticed the issue there as well) based on the last 50 rows:

``` python
def tare(df):
    return df - df[-50:].mean()
```

On rare occasions, this gave me an obviously wrong result (whole columns close to or equal to zero). negating the mean series and then adding

``` python
def tare(df):
    return df + (-df[-50:].mean())
# OR
def tare(df):
    return df + (-1.0 * df[-50:].mean())
```

will not solve the issue either.

Is there any other way I could/should write the function that will not cause the same issue?

Below is a full reproduction that works on my system.

``` python
import pandas as pd


def tare(df):
    return df - df[-50:].mean()


df = pd.read_csv('bug.txt')
for i in range(20000):
    try:
        assert tare(df).max().max() > 1.0
    except:
        print 'Failed on iteration {}.'.format(i)
        raise
```

Requires the file [bug.txt](https://github.com/pydata/pandas/files/105078/bug.txt) containing a subset of my data.

I haven't tried reproducing it using random data, but I guess that should be possible.

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.17.1
nose: 1.3.7
pip: 7.1.2
setuptools: 18.5
Cython: 0.23.4
numpy: 1.10.1
scipy: 0.16.0
statsmodels: 0.6.1
IPython: 4.0.1
sphinx: 1.3.1
patsy: 0.4.0
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.4.4
matplotlib: 1.5.0
openpyxl: 2.2.6
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.7.7
lxml: 3.4.4
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: None
Jinja2: None
```
