Mortal Kombat brought the dark side to fighting games in the early 90s, and once gamers started finishing off their opponents with brutal fatalities they couldn't get enough.
But, like most arcade machines, there seemed to be nothing more to the game than what's presented after you plunk in a quarter, until one player's button mashing led to an amazing discovery.
It seems the game's creator Ed J. Boon put a hidden menu in the original arcade versions of Mortal Kombat 1, 2 and 3, and whoever enters the right button "code" can do all kinds of cool behind-the-scenes stuff via the "EJB Menu".
All secret characters can be instantly unlocked, you can watch endings for various characters, see every fatality, run diagnostics and play virtually impossible Galaga clone the "Penacho/Miller Game".
Here's a video posted by discoverer YourMKArcadeSource showing all the cool stuff found in the EJB Menu:
-Via Sploid