title: Facebook iOS SDK: session invalid on iPhone after logging out of Facebook from a desktop
body: <p>I recently integrated Facebook support into an iPhone application that I work on.  I'm able to log the user in and make API calls fine, but I recently noticed some strange behavior.</p>
  
  <p>After the user logs in I store the access token and the expiration date in NSUserDefaults.  Before executing Facebook API calls I use [session isSessionValid] to make sure the session is valid.</p>
  
  <p>The problem occurs if the user logs into Facebook from a desktop and then logs out.  [session isSessionValid] still returns YES, but Facebook API calls fail with the error:</p>
  
  <p>"Error validating access token: This may be because the user logged out or may be due to a system error."</p>
  
  <p>Is there anything that I am doing wrong, or is this a Facebook bug?</p>