One of our office pranks was that if someone left their computer unlocked, we changed their wallpaper to a random picture of a goat. We would accuse them of doing all kinds of rude things with goats from that point onwards.
I also convinced a co-worker that his expensive new headphones were broken when I took control of his audio output and toggled the mute in one ear on and off randomly. I may be the devil. A related trick is have his computer play loud embarrassing sounds all day.