title: page tab's signed_request occasionally not populated
body: <p>I have a page tab application.  Sometimes when a logged in user clicks the tab name from the Facebook page, the form post "signed_request" data is being passed in is empty.  </p>
  
  <p>Any ideas as to how this can happen?  It is sporadic and doesn't seem to be locked to one specific user account.  I have oauth2 set to true in the settings, so according to Facebook's api documentation, I should always be passed signed_request in the form post data.</p>