If you visit http://w-beta.wmflabs.org/123, it displays an unstyled MediaWiki error page (once UrlShortener is actually deployed that url will display a different invalid short code message). This because MW is using relative paths to load.php (/w/load.php) which obviously don't exist on the short domain.
We need to output some kind of error message, is it possible that UrlRedirector can detect if it's on the short domain and redirect to the full one? I'm not sure if that's even possible with how the rewrite rules are set up.
Otherwise we could disable the normal MW output and just send a small plaintext error message.