It's been over 20 years since Mortal Kombat 1, 2, and 3 released in arcades and it's taken this long for someone to discover hidden menus in the game. The menus had originally been programmed and hidden in the games by Mortal Kombat co-creator Ed J. Boon.
Back in October, the Cutting Room Floor website revealed the secret menus existence (via ArsTechnica) across Mortal Kombat 1, 2 and 3, but YouTuber YourMKArcadeSource has brought them back to light with a video on how to unlock the menus on real arcade hardware.
In the Mortal Kombat 1 and 2 arcade cabinets, accessing the secret "EJB Menu" (Boon named the secret menus after himself) will allow you to watch various character ending, put initials directly into the leaderboard without earning your spot, run debug and diagnostic tests, as well as do something called "coin bookkeeping."
In addition to that, a "Hello" option is in the EJB Menu that lists a number of individuals Boon wanted to say 'hello' to.
The EJB Menu in Mortal Kombat 3 can be used to access quick fatality demos and play a Galaga-style game.