Enable Error Reporting

Although it may be turned off on commercial hosts and production sites (for good reason, so that users do not see the errors), these errors are one of your best tools for troubleshooting. To enable error reporting, temporarily edit your index.php file (normally located in your root directory) directly after the first opening PHP tag (do not edit the actual file info!) to add the following:

<?php

error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);

// $Id: index.php,v 1.94 2007/12/26...

http://drupal.org/node/158043/