System administrator Peter had some network troubles, but managed to resolve them by cross-referencing problem email messages to the dates in the change log—or journal of updates—he keeps.
I used that information to look through the log I keep of server and network changes and maintenance. It turns out that two months ago I made a change to the firewall rules, and the problem started the very next day. It was hard to see how this particular change would cause the machine to crash, but it was too coincidental, so I disabled that rule and restarted the firewall service. There have been no warnings, problems or crashes since. I hope you can see the value of good record keeping. If you are making changes to your network without keeping a detailed record of what you are doing and when you're doing it, it is going to be extremely difficult to diagnose problems later.
Although he works in a larger environment with more variables than a home computer user might have, the system is just as effective on a smaller scale. The first time I was introduced to a change log was through my grandfather's extensive computer notes and logging. An engineer, programmer, and avid computer enthusiast he kept meticulous notes about everything he had done to his computer and home network. His logs detailed dates of software installation and removal, registry changes, firewall tweaks, product keys, and times of routine maintenance. With such a simple system in place it becomes easy to find the ghost in the machine.
While my grandfather used a notebook, Peter moved on to using a blog so he could sort by date and tag the entries. If like you decide to go digital instead of keeping an paper change log, we've covered several techniques and programs here that could be useful. An extremely simple log could be created simply by making a text file and using the append feature in Quicksilver or for Windows users, Gina put together a handy script and tutorial that emulates the append functionality in Quicksilver. If you have a different technique or a handy tip for keeping a change log, share them with fellow readers in the comments.