Debugging a game is hard, or at the very least extremely tedious, and the bigger the game the more painstaking the task. You have to test every aspect of the game under every condition to make sure it all works properly. Sometimes to make life easier devs add in things like the Konami Code or Testinghall in Oblivion and Fallout 4. Mortal Kombat creator Ed Boon is no stranger to idea and put his own testing tool into the original Mortal Kombat games, a tool that players have only recently discovered after twenty years.
An article on Sploid shared video made by Mortal Kombat enthusiast and YouTuber YourMKArcadeSource showing off the hidden menus.
Basically, you access the menus by alternating between pressing the player 1 and player 2 block buttons a certain number of times, which can be kinda fiddly, and the secret “EJB” (Ed Boon’s initials) Menu pops up. The menus give the user access to various debugging options. Among the options are “Hello”, which is just a list of people Ed Boon wanted to thank and “Coin Bookkeeping”. The player can also see all the character endings, which could normally only be accessed by completing the story mode. The options in the EJB Menu for MK2 are the same, and it wasn’t until MK3 that Ed decided to add in some more features.
In MK3, along with the other menu options, the user can unlock all hidden characters, watch all fatalities and play a crazy difficult clone of Galaga.
No word on if anyone has, or is able to, access these menus on any console version of the games or if they exist in other Mortal Kombat sequels, but knowing how much developers like to hide things in their games there might be more secrets hidden, waiting for people to find them.