One of the common problems that I see with “SharePoint Experts” is when they decide to start monkeying with master pages, they usually blow things up. We had an instance where one of our “SharePoint Experts” decided to blow away a master page which resulted in the fun correlation error page. There are a few good ways to restore order to your world.
1. You take away their access to the site. 🙂 (kidding)
2. You can try to go in through SharePoint Designer to set it to a working master.
3. You can reset the master page with some P-Shell (kudos to Todd Klindt): http://www.toddklindt.com/blog/Lists/Posts/Post.aspx?ID=226
4. You can go directly to the change master page screen http://contoso.com/sites/_Layouts/ChangeSiteMasterPage.aspx
Yup.. Master Pages are fun.