title: What causes "requires valid signature" error when calling FQL from javascript SDK?
body: <p>My site uses FQL called from javascript as such:</p>
  
  <pre><code>FB.api({
      method: 'fql.query',
      query: 'SELECT uid, name, first_name, pic_big, pic, third_party_id from user where uid in (SELECT uid2 FROM friend WHERE uid1 = me()) ORDER BY name'
  },
  function(response) {
      if( response.error_msg ) {            
          alert("Sorry!  Something went wrong loading your Facebook friends:"+response.error_msg);
          return;
      }
      process_friends(response);
  });
  </code></pre>
  
  <p>}</p>
  
  <p>They get here after a normal <code>FB.login()</code> call.  This is working fine for most users.  But some users (or user sessions at least) consistently get the "requires valid signature" error and can't get past this, even after <code>FB.login()</code>'ing in again.</p>
  
  <p>What does that error really mean?  I'm guessing the oauth token is invalid?  But the token came from within the javascript SDK -- I never touched it, and didn't make the signature.  Could the user be in an inconsistent login state?  Insufficient app privileges?  Some strange cookie setting?</p>