Stefan Koopmanschap has a recent post to his blog about a small oversight (and gotcha other symfony developers should watch for) that he's made a few times when trying to work with a subdomain of his main symfony-powered website:

Quite often have I been staring at my virtualhost setup for the admin subdomain of this application, wondering why the hell it kept serving the frontend. I'd set the DirectoryIndex to backend.php, so why did it keep serving the frontend?

The culprit was something that's usually one of the first things set up when you load a new framework - the mod_rewrite rules in an .htaccess. Of course, since they were the default, things would get routed back to "index.php" instead of his "backend.php" as he wanted. To fix it, he moved these options out of the .htaccess and into the virtualhost configuration.