For fun, on our site one of the developers made it so that if you entered the Konami code, every image would be replaced by random cat pictures. This was immediately ripped out of prod a few weeks later when a client asked why there were a bunch of cat photos in their dashboard. Still no idea how they triggered it.
I once did this on an internal app, we had a hotkeys library installed, so I added the GTA cheats to raise and lower your wanted level. It just added a temporary notice in the top right of your screen, nothing major. Turns out that if you mashed the keyboard, the library didn't properly handle 6+ character combinations, and would fire the callback anyways.
We all felt very stupid and unprofessional.