Heroku - Application Error, but no error in logs

I'm hacking away at a Facebook app on Heroku, using Node.js. When I go to the app myself, it works exactly as I expect. When I send another developer/tester to it, the standard Heroku "Application Error" message is displayed. When I check the logs, there isn't even a log that the developer made a GET request for the app page, let alone an error.

He tells me that the last thing he sees in his browser status bar is "waiting for .herokuapp.com", which implies that Facebook got to the app, but the logs don't support this theory.

Is there another method of seeing Heroku errors? Is there something I have to turn on in order to see more? A setting I have to modify to let this other guy use the app?

Update: Some more looking around led me to check the network traffic. He's getting a 503 Service Unavailable, followed by the error page. Still wondering why this happens on his machine, but not mine.

Update: Inserted some error-checking code into the handle_facebook_request method.

req.facebook.app(function(err, app) {
    if(err !== null) {
        var errorMessage = "Error getting app: " + util.inspect(err['error']);
        console.log(errorMessage);
    }
    // rest of req.facebook.me, and rendering code
}

This gives me an "Unsupported get request", of type "GraphMethodException" and code 100. So now I have something I can look into! I wish that Heroku had at least mentioned that the request had been tried.