Google Site Maps 404 Error Status

Understanding the 404 error message

[graywolf]

I have been testing google sitemaps and it asked me to verify a submission last night. I received an error message directing to the troubleshooting page [google.com] with this message

We've detected that your 404 (file not found) error page returns a status of 200 (OK) in the header.
this configuration presents a security risk for site verification and therefore, we can't verify your site. If your web server is configured to return a status of 200 in the header of 404 pages, and we enabled you to verify your site with this configuration, others would be able to take advantage of this and verify your site as well. This would allow others to see your site statistics. To ensure that no one can take advantage of this configuration to view statistics to sites they don't own, we only verify sites that return a status of 404 in the header of 404 pages.

Please modify your web server configuration to return a status of 404 in the header of 404 pages. Once your web server is configured correctly, try to verify the site again. If your web server is configured this way and you receive this error, click Check Status again and we'll recheck your configuration.

I do have a custom 404 error page on the server. Can anyone explain how this is used to take advantage of another persons website?