Large number of likes but now realise it is to an invalid url

Question

Short of getting someone at Facebook to update the URL, the only option within your control that I could think of that would work is to create a custom 404 error page. I have tested such a page with your URL and the following works.

First you need to set the Apache directive for ErrorDocument (or equivalent in another server).

ErrorDocument 404 /path/to/404.php

This will cause any 404 pages to hit the script, which in turn will do the necessary check and redirect if appropriate.

I tested the following script and it works perfectly.

<?php

if ( $_SERVER['REQUEST_URI'] == '/%2Fviewtopic.php%3Ff%3D42%26t%3D370' ) {
    Header("HTTP/1.1 301 Moved Permanently");
    Header("Location: /viewtopic.php?f=42&t=370");
    exit();
} else {
    header('HTTP/1.0 404 Not Found');
}

?><html><body>
<h1>HTTP 404 Not Found</h1>
<?php echo $_SERVER['REQUEST_URI']; ?>
</body></html>

This is a semi-dirty way of achieving this, however I tried several variations in Apache2.2 using mod_alias's Redirect and mod_rewrite's RewriteRule, neither of which I have been able to get working with a URL containing percent encoded chars. I suspect that with nginx you may have better success at a more graceful way to handle this in the server.