Long ago, you might have seen this commercial: Snausages dog treat commercial
One of the most evil hacks I've ever seen was based on this. Long before OS X, on your Mac, about once an hour, a tiny dog icon peered out from behind a window, and said, "Snausages!" and then disappeared. You could set the hack to run rarely, and only when the user was typing or mousing busily for quite some time.
No, it's not evil because it interrupts you, it's evil because the first few times it happens, you're sure that you must have been seeing something. So then you start watching for it, waiting for it, and it senses that you aren't working, and doesn't show up.
Recently, I was told of a hack almost as evil: The Vetinari Clock: http://hackaday.com/2011/10/12/building-a-better-clock-to-drive-you-insane/
(The movie shows the effect exaggerated to an extreme.)
It keeps time well-enough on average, but each "tick" and second advancement might come a little too soon or a little too late, at random.