title: SettingWithCopyWarning, even when using loc (?)
body: <p>I get <code>SettingWithCopyWarning</code> errors in cases where I would not expect them:</p>
  
  <pre><code>N.In &lt;38&gt;: # Column B does not exist yet
  N.In &lt;39&gt;: df['B'] = df['A']/25
  N.In &lt;40&gt;: df['B'] = df['A']/50
  
  /Users/josh/anaconda/envs/py27/lib/python2.7/site-packages/pandas/core/indexing.py:389: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
  Try using .loc[row_index,col_indexer] = value instead
    self.obj[item] = s
  </code></pre>
  
  <p>and</p>
  
  <pre><code>N.In &lt;41&gt;: df.loc[:,'B'] = df['A']/50
  
  /Users/josh/anaconda/envs/py27/lib/python2.7/site-packages/pandas/core/indexing.py:389: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
  Try using .loc[row_index,col_indexer] = value instead
    self.obj[item] = s
  </code></pre>
  
  <p>Why does it happen in case 1 and 2? </p>