During summers in college, I worked for a small web dev company that had a private line into a client company's network (a microwave link between the buildings,

During summers in college, I worked for a small web dev company that had a private line into a client company's network (a microwave link between the buildings, encrypted and with firewalls on both ends.) One day, we completely lost the ability to log in from our building, and therefore, to do billable work for that client (though they had space for 2-3 of our employees to work in one of their offices.) The other company insisted they hadn't changed anything on their end, so it must've been something I did. I did research, packet logging, analysis, and so on, and couldn't find anything. This went on for days. Eventually I walked over to their server room, and found the screens said "Windows 2000" where they had previously said "Windows NT". Apparently they hadn't changed anything except for the operating system on the servers handling logins, which was clearly unimportant and not worth mentioning. Another five minutes of research and I discovered Win2K server used an additional service (triggered from their end) to log in, which was being blocked by their firewall and therefore never appeared in my logs. I opened one port on my end, their firewall guy opened one on their end, and a problem that was costing us half of our revenue was solved.