Setting a new column with the sum of two existing columns does not work correctly for large dataframes
Setting a new column with the sum of two existing columns does not work correctly for large dataframes:  X['C']=X['A']+X['B'] may or may not  work correctly for some runs and frame of size > 10000 X['A'] and X['B'] may contain any float number. This issue is absent in pandas 0.16.2.
I am running 64 bit python on Windows 10. See attached notebook for details:

[ReadDebug1.zip](https://github.com/pydata/pandas/files/95513/ReadDebug1.zip)

To reproduce:
### +++++++++++++++++++++++++++++++++++++

import pandas as pd
DataFrameSize=10001 ## will work with 10000 or less
XR = pd.DataFrame({'A' : pd.Series(1,index=list(range(DataFrameSize)),dtype='float32'),
                      'B' : pd.Series(2,index=list(range(DataFrameSize)),dtype='float32')})

def CleanDebug(X):  
    X.loc[:,'C']=X.loc[:,'A']+X.loc[:,'B']
    #X['C']=X['A']+X['B']  
    return X

for i in xrange(1000):
    print 'iteration ',i
    ry = CleanDebug(XR)  
    assert abs(ry.C.sum()-30003)<1   
