I was in college during the years leading up to y2k and supported myself at the time getting IT infrastructure ready. Some friends and I decided to write a "virus" that, on bootup, checks to see if the current date is in the first week of January 2000 and if it is and a backup of the fonts is not found (so it'll only run once) then it'll back up your fonts and alter the originals to replace the y character with the k. This affected everything system wide.
That created more chaos than anticipated.