And if you do a code change, make sure it is really funny, is easy to fix and discover and is invisible to customers. Like, inject a hilarious joke into the log files or something.
A coworker at a previous job altered the company's internal web application so that when one specific user was logged in, about 1/20 of the time it would load a hidden iframe that played Rebecca Black's Friday. This stayed in place for a long time because the user couldn't figure out what was going on and was too embarrassed to ask anyone else about it. Instead, they turned off the computer's sound and started listening to music on their phone.
I did have a friend that wrote a feature that would make ghosts flash across the screen. Long enough to see, but fast enough to make it hard to pinpoint for sure what you saw. It was locked to only happen when the "boss" used the app.
Kinda funny, no customer impact. Not sure what would have happened if he had a bug and made ghosts appear for everyone...
Right before leaving my last job, I updated an internal web-based tool to add a menu item called "Add Unicorns." It just used cornify to show unicorn images when clicked. But I also made it only show that menu item at random and somewhat rarely, so I heard that it took some people a while to notice, which was fun. To my knowledge, that change is still there!
A coworker of mine, definitely not me, left an html comment in a template for a page on a fairly big website that read similar to "Help I'm trapped in a website factory". So anyone perusing the generated html might read that. Similar to the xkcd pi joke: https://xkcd.com/10/
