You may have recently recieved an email from Google that looks like this:
When you follow the "Fix Index coverage issues" link, you see something like this:
And when you click on the error, you see something like this:
This filename "sitemap.html" error is due to a setting in one of the WordPress plug-ins. If your WordPress site was built by Acorn, we will be taking care of this for you at no charge. If we didn't build your site, contact your webmaster to have them help you fix this error.