If you haven't tested it, it doesn't work.

I got this from the Extreme Programming movement, but it's just as applicable to systems and procedures as it is to developing software. (Remember verifying backups ? That's an instance of this aphorism.)

This has an important corollary:

If you aren't checking it, it's broken.

For example, when you have a spiffy hardware RAID system you should make very sure that you do indeed get notified when a disk goes bad and your RAID ceases to be redundant. Otherwise, you will sooner or later appear in a comp.risks digest story.