Ultimate Mortal Kombat 3
Ultimate Mortal Kombat 3
Using Kombat Kodes (six digit numerical codes in sets of three, specified by Mortal Kombat iconographies) on the VS. character screen before a match, several messages can be conjured on screen when the match has begun. Several of these messages are developer messages and references to other games:

•999-999 = Displays a message indicating what version revision is being played.
•550-550 = Displays the message: "Go see the Mortal Kombat Live Tour!", advertising the 1995 martial arts theatrical stage show of the same name.
•122-221 = Displays the message: "Skunky! - E.F." which assumedly is an inside joke from a developer playing on the "Toasty!" gag by programmer Dan Forden.
•987-666 = Displays the message: "Hold flippers during Casino Run". This is a reference to Jack*Bot, an arcade pinball cabinet produced by Williams Entertainment who has worked with Midway on the home console ports of Mortal Kombat titles.
•004-400 = Displays the message: "Whatcha gun do? - E.B", a joke message from Ed Boon that possibly pokes fun at the fact of Mortal Kombat being a fighting game.

The Sega Saturn release of the game also features two exclusive developer messages:

•200-002 = Displays the message: "It should work!? - P.A.B."
•300-003 = Displays the message: "PUB! - D.K.P."
Contributed by Regen-33
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In the Arcade version's Attract Mode, Rain can be seen fighting Shao Khan for about a second. Rain was not a playable character in this version but was added to stir up rumors of his alleged inclusion.

Upon closer inspection it can be noted that Rain's name within his health bar is copy-pasted in a different font than what is standard for character names during matches.
Contributed by Regen-33
In the attract mode for the arcade version, there is a one second clip of Rain fighting Shao Kahn, despite the fact that Rain does not appear in this version. This could either mean that Rain was meant to be added to the game or that he was placed there to make fans speculate on if he's real or not.
Contributed by gamemaster1991
By using a Game Genie on the Genesis, the player can use Chameleon. He is clearly unfinished as he has no combos, special moves, or fatalities, uses Scorpion's sprites (with the sprite changing to Rain's when a decapitation fatality was performed on him), and the announcer does not say his name. This hints that he was planned to be put in the game before he was debuted in Mortal Kombat Trilogy.
Contributed by gamemaster1991