in cms/code/model/ErrorPage.php at line ~ 170 :

$this->extend('getDefaultRecords', $data);

what does it exactly?
Thanks.timo

whole function:

protected function getDefaultRecords() {
		$data = array(
			array(
				'ErrorCode' => 404,
				'Title' => _t('ErrorPage.DEFAULTERRORPAGETITLE', 'Page not found'),
				'Content' => _t(
					'ErrorPage.DEFAULTERRORPAGECONTENT', 
					'<p>Sorry, it seems you were trying to access a page that doesn\'t exist.</p>'
					. '<p>Please check the spelling of the URL you were trying to access and try again.</p>'
				)
			),
			array(
				'ErrorCode' => 500,
				'Title' => _t('ErrorPage.DEFAULTSERVERERRORPAGETITLE', 'Server error'),
				'Content' => _t(
					'ErrorPage.DEFAULTSERVERERRORPAGECONTENT', 
					'<p>Sorry, there was a problem with handling your request.</p>'
				)
			)
		);

		$this->extend('getDefaultRecords', $data); // ?????

		return $data;
	}

It is a hook point.
It enables Decorators/Extensions to run if they're using that hook.
You can learn more here: http://beta.docs.silverstripe.org/en/developer_guides/extending/extensions/#modifying-existing-methods

Nightjar!
Clear answer!
thanks.timo.