You kids may have heard about that whole "Y2K" thing, where all the date-manipulating code in the world was going to go sideways once '99 turned into '00. Planes were going to fall from the sky, ATMs were going to jackpot cash into the street, nuclear power plants were going to melt down, dogs and cats were going to move in together, and so on. People were feverishly spending their time digging through any code that cared about what time it was. Warnings were added to compilers. Anyone who wasn't planning to convert their money to krugerrands and ammunition was a fool! It was a full-fledged nerd panic.
So I have this program Dali Clock that maybe you've heard of. And for many moons, Dali Clock had been distributed on the CDs along with just about every Linux distribution in the world.
And I had an Idea.
So, in late 1998, I hid a prank in it. I had to start on this project very early in order to ensure that there would be time for the code to make it out into the world: it would take at least a year for the release cycles of the various distros to pick up the new version and get burned and shipped out. Though, I was certainly helped along by the fact that everyone was in a Y2K-addled upgrade fever in 1999. It would simply not do to be running a software release from 1997, oh no. So I got my code out there, and nobody noticed it amongst the set of other diffs in the release.
The prank was this:
If you happened to be running Dali Clock exactly at midnight on Jan 1, 2000, it it would start running "backwards" -- at midnight, the digits would mirror right-to-left. But this would only happen if Dali Clock had been launched in 1999. If you quit and restarted it in 2000, it was all back to normal. The source code in question also avoided using any obviously greppable constants like "99" or "2000" or "946713600" that might have set off alarms.
I got a bunch of almost-hate mail between 12 AM EST and 2 AM PST from people who had dug into the code and then eventually realized that it had been intentional... and that it had been lying in wait there for years...
It was glorious.