If you're developing ASP.NET applications on Windows Vista that are utilizing the Visual Studio-provided web server (a.k.a., Cassini) then you may have run into an issue where, when attempting to debug your website, Visual Studio immediately drops out of debugging once your application starts.  That is, you press F5, the browser pops up and displays the website and Visual Studio stops debugging and drops back into its standard development mode and the browser continues on running.  Your breakpoints are no longer active and you essentially can't utilize your development environment to troubleshoot issues with the site.

This has been happening to me of late.  I don't usually use the debugger in that sense to 'walk through' my code for a variety of reasons, but tonight I needed to.  Sure, I could attach to the process (WebDev.WebServer.exe), but that's a pain and I get tired of constantly attaching and detaching.

The culprit in my case was that http://localhost was in my Local Intranet zone in IE and, using IE 7 and Vista, I'm not authorized to debug intranet sites.  The solution is an easy one: move http://localhost to the Trusted Sites list in Internet Explorer.  Now the debugger attaches and remains attached, greatly simplifying the debugging process.