title: Instability of pandas dataframe calculations
body: <p>I'm wondering whether anyone has seen this problem with Pandas before.  Basically, I'm trying to add, multiply, and divide DataFrames element-by-element (all the frames have identical indexes and columns), but Pandas is spitting out different results for the same calculation performed successively.</p>
  
  <p>An image of some example output is shown below.  I've used <code>.values</code> in the code below because for display purposes, but the instability can happen when using <code>.add()</code>, <code>.mul()</code>, or <code>.div()</code>.  For example, if I repeatedly enter <code>N11.add(N00)</code>, I usually get the correct answer, but occasionally (every 4th or 5th time), I get a DataFrame filled with 0s.</p>
  
  <p><a href="https://i.stack.imgur.com/dWaMl.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/dWaMl.png" alt="enter image description here"></a></p>
  
  <p>If it matters, I'm on Windows 10 using an Anaconda distribution of Pandas 0.17.0 (with Python 2.7.10 on Spyder 2.3.7).  The frames that I am working with are large (6856 by 12511).  Has anyone else encountered this problem?  Is this a known issue or am I doing something wrong?</p>