The web.config file is a hell of a place to store your settings. The downside is that you have to be very carefull about what you type. The whole file is parsed before the app is started. When an error is found, the app will not start. Trying to debug this from VS, by pressing <F5> will present you with a very serious message:
Sounds bad. But when you start the app without debugging, by pressing <ctrl F5>, asp.net will tell you what's wrong in every detail:
Peter