Or, why do developers have too much free time?
Easter egg are almost a tradition nowadays in every sort of media, and how could it be otherwise? Secrets are always a doozy, even if you're only finding out about them from someone else (with many creators demonizing reddit and datamining for ruining the "experience") and they generally are less serious than the context the media presents itself in (wait what the fuck did I just write?)
Anywayy.. there's a few popular codes that found their way into many places just by virtue of being popular, let's explore a few:
A lot of games and other interactive media ship in bundles, usually whatever archive you download them off your platform of choice. Sometimes these archives are full of data that's unused or discarded, but sometimes it's hidden on purpose!
Popular examples of this are Undertale where a lot of the unused data and code actually does stuff when interpreted a certain way.
You know I'm too old when I need to explain this one. So basically early videogames did not have the luxuries of today when it came to input devices, plus were mostly a single player experiences only, therefore many allowed cheating in the game itself through something that was equally hidden yet accessible with controller inputs: sequences of button presses.
Konami games in particular started sharing a sequence that became so popular that other developer started integrating it into their works as a easter egg!
The sequence goes: Up Up Down Down Left Right Left Right B A (and an optional START where applicable).
If you want to play around, try it in your favorite website or just check all the places you can do it courtesy of Wikipedia: