I hadn’t seen the Clippy parody. I wrote a macro which I’d add to the default document template at unoccupied workstations with similar behavior - he’d pop up at random intervals and say “This document is no good.” Very rarely, he’d bounce around the screen, pong style, and cycle through random animations (and there are lots of them). I think I was the only one who was sad when Clippy was removed from Office, as that prank was now worthless.
A co-worker at an old job pulled a great goating prank - he installed a windows service which allowed remote control (via browser) of the audio playback volume. His cube was in the same room as the victim, who loved to listen to techno on his headphones. The perpetrator would slowly tweak the volume louder, louder, REALLY LOUD… then when the victim lowered the volume, he’d keep making it quieter and quieter. He had fun with that for months - when the victim figured that out, he hacked the other guy’s website. It was the start of an entertaining, sort-of-friendly-ish war.