Most of this chapter, quite unintentionally, turned into a breakdown of disks and filesystems. This is because, traditionally, disks were the most likely thing to go wrong in your system. Recently, disks don't die anywhere near as much, because the advent of cheap and commercially available SSDs has removed the "spinning rust" from a lot of systems.
That said, data can, and will, randomly disappear from your life.
Backup! Backup! Backup!
It doesn't matter how many times I say it—some of you will still read those words and think "yeah, I should do that" with no intention of ever bothering to set something up. For your own systems, that's your choice, but you might at least consider it for those boxes you manage, as it'll only make your life easier (and you the hero) when you break out the backups after...