If there is one thing the Mortal Kombat games have been known for, it's the huge number of secrets that Ed Boon and the crew loved to hide in the early arcade games. Back then the internet wasn't as huge as it is now, so there was a real mystique behind these secrets. Think of the conditions for unlocking the Reptile fight in Mortal Kombat one.
Now imagine how rare it would be for someone to do all of that without knowing it. You relied on the rumors from friends and other players to discover this stuff. I remember being in an arcade and people saying that you could unlock Johnny Cage in MK3 by holding down the run button and tapping down three times in the graveyard in front of the grave that says "Cage". Of course that wasn't true.
At this point, everyone has pretty much thought that everything there is to discover. Until now that is. zpaul2fresh8 has discovered some secret menus in Mortal Kombat 1, 2 and 3. These menus, called the "EJB Debug Menu" let you go into special menu's that will allow you to do some pretty cool stuff including seeing a special "Hello" message from Ed Boon, being able to see any characters ending at any time and editing the initials on the most number of wins screen. The fact that this stuff was hidden away for over 20+ years is incredible and really makes me sad that this kind of stuff would pretty much be impossible now.
Here is how you unlock the secret menu for each game. P1 and P2 represents the block button