It’s very handy to enable debug mode at developement stage. In order to do that you have to modify magento’s index.php file.

Change lines

if (isset($_SERVER['MAGE_IS_DEVELOPER_MODE'])) {
Mage::setIsDeveloperMode(true);
}

to

//if (isset($_SERVER['MAGE_IS_DEVELOPER_MODE'])) {
Mage::setIsDeveloperMode(true);
//}

and

#ini_set(‘display_errors’, 1);

to

ini_set(‘display_errors’, 1);

Now you if an error occurs magento will show its stack trace instead of report page.
And will log debug info and exception information in magento’s var/log directory.