Sometimes it's so tempting to not test your changes. You're in a rush, it's just a config change, it's a small change, or maybe you know it has no side effects. And if it's late on Friday or you're about to go on vacation and who has time? What could possibly go wrong?
Everything, that's what. If you've been around for a while you might remember the entire SJC datacenter going dark over a 20 minute period. It was just a simple change. To an IPTABLES config. That got distributed far more widely than it should have. With no oversight. By mistake.
The bottom line is that you do need to test all changes. Or at least run the automated tests you already have (you do have them, right?). And get code reviews. Even on the simple things.
But just in case you need it, here's a quick set of common excuses for not testing things. What's your favorite excuse?