Know How To Confirm It Works
You've just finished building a new feature and you're ready to launch. Great!
So how do you know it works?
At this point you should make a list (seriously, write this down) of what should be happening when your new feature (or bug fix or whatever) is in place. What metrics should be moving? Should CPU load go down? Will you stop seeing certain errors?
Also think about what shouldn't be happening. Is database load spiking? Are new errors popping up? Did load times just increase?
If you don't have visibility into these things before you launch, then you probably aren't ready to launch.