A coworker once set up my computer to audibly read from a file, which he was editing on the fly. He had it mostly read numbers over the course of an afternoon, until he overplayed his hand and I realized it was him. I promised him revenge, and I delivered.
Our small area had office music, and "DJ of the day" rights alternated between the half dozen of us in the space. My coworker (who I will call Allan because that's his name) repeatedly betrayed the sacred trust placed in him on his DJ day—namely, don't play intentionally shitty music. So I decided my revenge would involve the office music.
We streamed our music over AirTunes, which has a "feature" I was quickly able to exploit; namely, that if someone else plays their music, and sets their volume at zero, it won't let anyone else connect, but otherwise appears like it's not in use. So Allan would have great difficulty in connecting to the music. When he asked anyone else to try, I'd just hit stop, and they'd have no trouble at all. He'd repeatedly go unplug the AirPlay receiver, but I could see the startup light from my desk, so I could generally hit play at exactly the right moment to scoop it before him. Eventually, I'd let him start playing, if I was in a generous mood that day. This progressed for several weeks, before I decided it needed to go further.
My command line abilities are very limited, but a friend gave me some commands for remotely ending processes over the network, which I started randomly using to exit Allan's iTunes if he was playing any office music. This would really make him rage, and I even enlisted someone else in the office to do it when I wasn't there so it didn't seem suspicious that iTunes perfectly whenever I wasn't in the office.
My favourite part of fucking with him though was one small detail: I knew when I started crashing his applications that he'd search out help online, and the main place I'd seen him searching was macrumors.com. So I set up a question detailing the exact issues he'd been having. Then, I coerced a friend who was active on the site (so had a notable history of solving problems) to respond that the problem appeared to be a conflict with another application, and he'd solved the problem by uninstalling it. The software in question was an application I knew Allan loved and would never uninstall. So, as soon as his computer started acting up, he immediately went online and found out the bad news.
Anyway, this went on for about 2 years, until he left the company. Honestly, persistent bugger to keep trying for so long despite all the issues. I never did tell him, because after a while it felt like it was more cruel than a justified level of prank retaliation. But if you're reading this Allan, I'm not sorry. Your music was a new level of shittiness.