I love thinking about good pranks. I never do any of them, but I like making them up, and I talk a good game. Anyhow, here’s one for ya — the auto-replace feature in Microsoft Word. It’s just begging to be abused.
One day, when your co-worker is out for a few minutes, slip over to their desk. Go to Tools, Autocorrect, and start adding a few entries. For example, change “proactive” to “practice”, or “tone” to “stoned”. Next time your co-worker happens to type those words, they’ll be automatically changed. This all happens very quickly, it’s very easy to miss. If you make the autocorrect subtle enough, odds are good it will slip right past. Even if it noticed, most users don’t know this feature exists, and will have no idea what on earth could be doing it. Since it probably won’t be noticed until weeks or months until after you made the change, it will be hard to connect it to you.
It’s like being evil Letterman.
Have fun!