Since error logs are one of the most common tools to diagnose issues, tail -f logfile can be a very helpful command - if the behavior can be repeated. Many errors are logged to /var/log/Xorg.0.log and ~/.xsession-errors while running X; errors in X itself to the first, and errors in programs running inside of X to the second. While opening the log file in a text editor may be possible, watching the log file in an open terminal with this command allows the user to see the last ten lines logged, followed by any errors as they are added to the log in realtime, allowing a user to more easily associate error messages with the issue needing troubleshooting. A user can simply 'tail' a log file, and then try to reproduce the error, watching for new lines to appear. Since this command will run forever, Ctrl-C can be used to return to a prompt.
Other logs can be helpful to watch, such as /var/log/dmesg for messages from the kernel, for example, relating to USB devices as they are plugged in, hard disk failures, and other hardware issues. Many times just searching the internet for a particular logged error can point a user to others who have documented the same issue, or help provide specifics to those trying to help.