title: Can't add self as subview
body: <p><img src="https://i.stack.imgur.com/1QAli.png" alt="Image of Crashlytics back trace from users in the field"></p>
  
  <p>We use <code>Crashlytics</code>, 30+ users have seen this crash. this crash log is from users in the field.  we have never been able to reproduce it.  This is running on <code>iOS7</code>.  No clue what is causing this, as you can see there is nothing in the call stack relating to our app.  Anyone else see this or solved the issue?  thanks!  Please don't ask me to post code (see comments above).</p>