I always remember the day one of our major customers came to us, furiously demanding to know what we had done to block all their users. Their number of "visits" had dropped by about 80% and they were sure we had done something to prevent their users accessing the site. They also wanted us to track down one rogue user that now accounted for almost all the traffic to their site. That user, it turned out, had the IP address of our new proxy server. Flawed is a key concept in log analysis.