Here's one for you:

Trying to open any page with Sharepoint Designer results in

"Web Site Editing is Disabled. This web site has been configured to disallow editing with SharePoint Designer. Contact your web site administrator for more information"

The behavior is by design for the Root Web of a Project Server Site Collection (Template PWA#0)  and implemented through the ONET.XML Project Element http://msdn.microsoft.com/en-us/library/ms438080.aspx DisableWebDesignFeatures.

Reason?

An administrator or designer can accidentally break the whole functionality of the site by incorrectly modifying pages at the root web of a Project Site.