It is not unheard of to come across a solution where use of asp:Placeholder and asp:Panel (or some other container) Visibility is employed to show/hide certain details from the user especially when when implementing "simple" Authorization/Permission features...
I've come across a number of issues in the past where the cause of the issue is that someone hand edited the web.config and resulted to a not well formed XML (missing tags, unclosed quotes et al). The recommended way of doing it is to minimize these issues...
An interesting point about the ASP.NET Development Server (previously named "Cassini") was raised hours ago by Alvin, a coworker of mine. Forgive my ignorance but it seems that the said builtin server was constrained to be accessed only with localhost...