I encountered an issue today while trying to use the Server.MapPath method in an asp.net web page.  When trying to use Server.MapPath with a virtual directory in IIS (outside the web site) I was receiving an System.InvalidOperationException: Failed to map the path error.  Turns out, since I was running the web site as a file system project, and using cassini, this error was being generated.  By moving the project to IIS and running the site under IIS and not cassini, the error went away.