Another aphorism of system administration
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.