I am using the Relaxation 3-column theme, which comes with a custom 404 error page. I have also copied that page per Yahoo's instructions and placed it at the root of my site.
When you attempt to access a non-existent url on my site, such as http://rulebrittaniea.org/error, you get the 404 page in my root directory. However, when you try to access a non-existent url that is in a directory below the root, such as http://rulebrittaniea.org/index.php/2006/02/20/error/, you get Yahoo's 500 error page.
I just got off the phone with Yahoo's totally retarded customer service team, who told me that the custom error page is being redirected to the 500 page because of the index.php extension — the guy's exact words were that "There are certain parameters the error page looks for, but php does not have those parameters, so the custom error page fails."
His suggestion was to get rid of the index.php extension on all my permalinks, but because Yahoo does not offer htaccess, this is the only way I can offer pretty permalinks (not to mention it is sort of a pain in the ass to change permalinks when you know people are linking to pages on your site).
I don't buy the "php parameters" explaination, either, because if you use the search function to search for a bogus term, you get the correct 404 page, with the index.php extension in the url, like here: http://rulebrittaniea.org/index.php/?s=bogus.
So, am I retarded, or is Yahoo?