Today, if a browser makes a HTTPS request to a site with a certificate error, and the user picks "No" when Fiddler asks whether to accept the Certificate Error, it is very difficult to figure out where the HTTPS request made in error came from.

It would be cool if instead of simply closing the TUNNEL connection, Fiddler instead had an option by which the server connection was rejected but the client connection to the Tunnel got a 200 OK but was connected to a special "DEAD" pipe that returned HTTP/503.

That way, the client could make its HTTP requests to the dead pipe (whose URL and Referer header might reveal from where the request came) allowing the user to debug, but overall security would be maintained (no network connection made insecurely).