One of my earliest coding jobs was at a firm that did outsourced accounting on some off-brand minicomputers. They cheated their customers and treated their staff like slaves. I was there to do some routine coding, but knew how to write driver code, so I did that in my spare time. I noticed something about the disk drives they were using: they had a resonant frequency. So I wrote some code to do head-seeks that took as long as the resonant period. I buried it deep, obfuscated it, triggered it by a random event that would occur on average every few days, and activated it when I left. I later heard from a colleague that they started having head crashes and guess what, they’d never tested their backup/restore process end-to-end.

I am pleased that the statute of limitations has long ago elapsed on that little adventure.