Custom error pages in joomla

Joomla! uses the templates/system/error.php file to handle several HTTP Status errors, including "403 Forbidden", "404 Not Found", and "500 Internal Server" errors. You can style the error results, if desired.

It is important to understand that error.php is an independent file from Joomla!. Plugins do not run on the file. You cannot include modules or use <jdoc:include> statements.

Overriding the System Error Results

To override the system error results, copy the templates/system/error.php file into your templates/<template-name> directory.
If it finds one, Joomla! will use the error.php file from the current template, in place of the system file.
You can format the page, as desired, to match your template.

Overriding the System Styling

If you want to change the styling, copy the templates/system/css/error.css file into your templates/<template-name>/css directory.
Next, update your templates/<template-name>/error.php file to reference the new location of the stylesheet by changing this line, accordingly:

<link rel="stylesheet" href="<?php echo $this->baseurl; ?>/templates/system/css/error.css" type="text/css" />

Then, simply change the error.css, as desired, for your styling requirements.

Customizing Error Messages

You can add conditional logic to vary the message returned, dependent upon the specific error code.
Here is an example of how to trap a 404 error and provide a custom message.

<?php if ($this->error->code == '404') { ?>
        <div id="errorboxheader">Page not found</div>
                <div id="errorboxbody"><p>Sorry! That page cannot be found.</p>
                </div>
        </div>
<?php } ?>

HTTP Status Code

When a request is made for a page on your site, the server returns an HTTP status code in response to the request. Joomla! returns a '200 - the server successfully returned the page' for error pages. This is problematic for those working with Google Webmaster Services and trying to get a sitemap resolved.
If you want Joomla! to return a status code for the error, you can do so by adding logic before the DOCTYPE line, as follows:

<?php 
if ($this->error->code == '404') {
        header("HTTP/1.0 404 Not Found");
} ?>

More HTTP Status Code Information

• Joomla! Documentation: Create a Custom 404 Error Page
• HTTP/1.1 Status Code Definitions
• Google Webmaster - Analyzing Crawl Errors

Using Theme Header and Footer on Standard Error Page

If you want to see the error page in theme design and don't like redirecting to error page URL or duplicating HTML in the error page template, here is a way to apply your theme template to the error page.
First, put the following code in templates/<template-name>/error.php:

<?php
// no direct access
defined( '_JEXEC' ) or die( 'Restricted access' );
 
include dirname(__FILE__) . "/index.php";
?>

Then make the following edits to templates/<template-name>/index.php::
1. Find the following code in index.php

<jdoc:include type="head" />

and replace it with the following

<?php if (!$this->error->code) : ?>
<jdoc:include type="head" />
<?php else : ?> 
<title><?php echo $this->error->code ?> - <?php echo $this->title; ?></title>
<?php endif; ?>

2. Find the following code in index.php

<jdoc:include type="component" />

and replace it with the following

<?php if ($this->error->code) : /* check if we are on error page, if yes - display error message */ ?>
<p><strong><?php echo $this->error->code ?> - <?php echo $this->error->message ?></strong></p>
 
<p><strong><?php echo JText::_('You may not be able to visit this page because of:'); ?></strong></p>
<ol>
<li><?php echo JText::_('An out-of-date bookmark/favourite'); ?></li>
<li><?php echo JText::_('A search engine that has an out-of-date listing for this site'); ?></li>
<li><?php echo JText::_('A mis-typed address'); ?></li>
<li><?php echo JText::_('You have no access to this page'); ?></li>
<li><?php echo JText::_('The requested resource was not found'); ?></li>
<li><?php echo JText::_('An error has occurred while processing your request.'); ?></li>
</ol>
<p><strong><?php echo JText::_('Please try one of the following pages:'); ?></strong></p>
<p>
<ul>
<li><a href="<?php echo $this->baseurl; ?>/index.php" title="<?php echo JText::_('Go to the home page'); ?>"><?php echo JText::_('Home Page'); ?></a></li>
</ul>
</p>
<p><?php echo JText::_('If difficulties persist, please contact the system administrator of this site.'); ?></p>  
<?php else : ?>  
<jdoc:include type="component" />
<?php endif; ?>

Now your theme template is applied to error pages too.
Note: Module includes in template will not work on error page created by this method (but will work on other pages).

source: http://docs.joomla.org/Custom_error_pages