In some circumstances a file is not supposed to exist, but if it does come into existence it’s a bad sign.

In some content management systems you can create code so a URL appears only when something has failed. This filter is designed to monitor for this type of event.

This filter can also be used when a site is “down” and you want to know when it comes up again. As long as the URL is unreachable, nothing happens, but you’ll be alerted when the URL appears.