Today I was experiencing unexpected 404 "The resource cannot be found" errors when working with Visual Web Developer 2005 Express Edition beta 2 in combination with the ASP.NET Development Server. No matter what I tried, and what file I tried to open, I kept getting the same error.
I was just about to get very angry, after trying to change about every property on my project, web server and firewall when I noticed an additional file in the root of my site called app_offline.htm. It turned out this file was the culprit. I have no idea how the file got there, but I think it has been put there by Visual Web Developer somehow.