title: Mixed types when reading csv files. Causes, fixes and consequences
body: <p>What exactly happens when Pandas issues this warning? Should I worry about it?</p>
  
  <pre><code>In [1]: read_csv(path_to_my_file)
  /Users/josh/anaconda/envs/py3k/lib/python3.3/site-packages/pandas/io/parsers.py:1139: 
  DtypeWarning: Columns (4,13,29,51,56,57,58,63,87,96) have mixed types. Specify dtype option on import or set low_memory=False.              
  
    data = self._reader.read(nrows)
  </code></pre>
  
  <p>I assume that this means that Pandas is unable to infer the type from values on those columns. But if that is the case, <strong>what type does Pandas end up using for those columns</strong>?  </p>
  
  <p>Also, can the type always be recovered after the fact? (after getting the warning), or are there cases where I may not be able to recover the original info correctly, and I should pre-specify the type?</p>
  
  <p>Finally, how exactly does <code>low_memory=False</code> fix the problem?</p>