Imagine the following:
You're responsible for an application which is used by dozens of people in your company, and that application is running on a webserver, which you don't have physical access to.
Now imagine there's a sysop, performing updates on the machine on a regular basis, but never testing if the applications running on that webserver are still working after that.
Then, one cold and cloudy morning, you arrive at your company, sit down at your desk and boot your computer, and only a few seconds after that you hear the phone ringing. Yes, it's one of the people who use to work with your application, and he's quite worried, because