When I moved a bunch of URLs around for my new site, I knew I’d probably miss a couple of redirects and serve up 404s (file not found) for things I shouldn’t. I created a little plugin to help me with this.
404 Notifier keeps a log of 404 (file not found) hits on your web site, along with the referring URL if there is one. You can receive e-mail notifications of these hits, or you can subscribe to an RSS feed of them (recommended).
The download is available on my WordPress Plugins page.
If you have any trouble with this, please open a thread in the WP Support Forums and send me the link.