A couple of days back we implemented customized 404 error page. Our main purpose was to provide some useful information to the visitors visiting a missing page (with contents like Page Does Not Exist. or No posts made) and make them stay. This is a necessity for all blogs because it may so happen that another blog may have linked to a post or page, which you have deleted over the time. So visitors coming to that page will get a blank page and run away.
The problem with creating a customized 404 error page at Blogsome
For those of you having blogs hosted in your own domains, please check this page to customize your 404 page. Free blog hosting platforms like Blogsome do not permit you to customize your template as per your wish. For example, in case of Blogsome you cannot insert php, run your own sql query, use all the wordpress template tags, etc. And I think this is pretty much understandable as their main objective is to provide an uninterrupted blogging service. (In this regard its worth mentioning that Blogsome has surpassed all my expectations.) So it is not possible to customize 404 pages as stated here.
The solution
We eventually found a solution (example). Let me share it with you. Generally in a 404 error page we get to see content like Page Does Not Exist. or No posts made What we will do is check the content every time the page is loaded and if we get contents like that, we will replace it with our own content. This Blogsome tip is search engine friendly too i.e. the search engines won’t get puzzled and penalize you. Here are the steps to do that:
1. Open the blog control panel.
2. Go to Manage and click on the Files tab. Now you need to edit the index.html file which is the main template file of your weblog.
3. Try to locate the {content} tag inside the index.html file. You can find it just below the <body> tag.
4. Replace the {content} tag with this:
{capture name=capname}{$content}{/capture}
{if $smarty.capture.capname == ‘Page Does Not Exist.’ || $smarty.capture.cont == ‘No posts made’}
OOPS! Page not found (404 error)
{else}
{$content}
{/if}
Thats it! Now save the file and open a missing page like http://your_blog_url.blogsome.com/404 (replace your_blog_url with your blog url) and see the result. If your blog is not hosted in Blogsome, try out the above code if smurty tags are supported or convert the code into php.
Tweak the code as you wish. But please do not forget to give the credits if you are not that lazy 
Happy blogging!!
Hi!
Nice work! Just implemented the code on my site too..