spurious SettingWithCopyWarning 
I'm getting spurious warnings on some old code that I'm running with new pandas. You can replicate by doing something like this (you have to take a subset of the data first, that's the key)

```
import pandas as pd
from pandas.core.common import SettingWithCopyWarning
from string import letters
import warnings
warnings.simplefilter('error', SettingWithCopyWarning)

def random_text(nobs=100):
    df = []
    for i in range(nobs):
        idx= np.random.randint(len(letters), size=2)
        idx.sort()
        df.append([letters[idx[0]:idx[1]]])

    return pd.DataFrame(df, columns=['letters'])

df = random_text(100000)

df = df.ix[df.letters.apply(lambda x : len(x) > 10)]
df['letters'] = df['letters'].apply(str.lower)
```
