When your app is crashed, out of resources, or misbehaving in some other way, Heroku serves an error page. (You can see what this page looks like here.)

To find out details on the error, check your logs:

$ heroku logs
2011-03-01T16:16:29-08:00 heroku[web.1]: State changed from starting to crashed
2011-03-01T16:16:59-08:00 heroku[router]: Error H10 (App crashed) -> GET crasher.heroku.com/ dyno=none queue=0 wait=0ms service=0ms bytes=0

In this example, we see that the router tried to serve a page for the app, but the web process is crashed. Check your app’s backtrace in the logs to find out what you need to do to fix the problem.

See the complete list of error codes.

You can customize the look of the error page using the Custom Error Pages add-on.