I had an issue recently with in-line java-script not being rendered using AJAX ASP.NET. So I fire up Fiddler to see whats going on. Only when ever fiddler is open the app no longer works, it throws a WebException complaining that the connection was reset or aborted.
We are using Integrated Authentication on a separate server that the app talks to via web services. It turns out by default Fiddler doesn’t get along with the way .Net Web Services authenticate. It closes any connection that returns a 401.
This article explains the problem and the fix (which is to add a custom rule to fiddler to stop it closing the connection)