3. The Secret Debug Menus Undiscovered For Over 20 Years - Mortal Kombat
Though the original three Mortal Kombat games were released in 1992, 1993, and 1995 respectively, only in 2015 was a secret debug menu hidden within the arcade versions of the games uncovered by fans.
The menu, named "EJB" after Mortal Kombat creator Ed John Boon, could be unlocked for the original game by inputting a 27-button code which alternates between the machine's "P1" and "P2" block buttons. The sequence for the first Mortal Kombat is as follows:
P1 (x5), P2 (x10), P1 (x2), P2 (x1), P1 (x2), P2 (x3), P1 (x4).
This menu, which is only accessible on the arcade cabinet versions of the games, allows "players" to run diagnostics on the unit, make small changes to its presentation, view the various character endings, and even edit the high score table.
The EJB menu for Mortal Kombat 3 actually goes much further, including a Galaga mini-game and allowing players to unlock the playable character Smoke.
The Ultimate version of the game, however, even allows you to watch a demo sequence of all the game's Fatalities, Animalities, Babalities, and so on.
Needless to say, the fact that this went undiscovered for so long probably saved arcade operators more than a few grey hairs over the years.