On the VS screen (in 2 player mode), you can cycle through the 6 symbols by hitting Low Punch, Block, and Low Kick on both sides. Cycle backwards by hitting UP and the button. The numbers are the number of times you hit the button.
Kombat Kodes:
999-999 - Show what revision you have.
010-010 - Throwing Encouraged
100-100 - No Throws
020-020 - No Blocking
987-123 - No Power Bars
300-300 - Silent Kombat
788-322 - Quick Uppercut Recovery
033-000 - Half Energy for Player 1
000-033 - Half Energy for Player 2
707-000 - Quarter Energy for Player 1
000-707 - Quarter Energy for Player 2
044-440 - Sans Power
688-422 - Dark Fighting
444-444 - Randper Kombat (random morphing) (or 460-460)
985-125 - Psycho Kombat (dark, randper, no meters, no block, quick uppercut)
466-466 - Unlimited Run
642-468 - Galaga (shows UKK screen on XBox 360 instead)
Kombat Zone:
091-190 - Bell Tower
077-022 - The Bridge
933-933 - Scislac Busirez (Ermac's Portal)
666-333 - Graveyard
330-033 - Jade's Desert
004-700 - Kahn's Kave
880-220 - Kahn's Tower
600-040 - Kombat Temple
050-050 - Noob's Dorfen (Bell Tower)
820-028 - Pit III
002-003 - River Kombat
343-343 - The Roof
666-444 - Scorpion's Lair
123-901 - Soul Chamber
079-035 - Street
880-088 - Subway
Text: (displays a little message)
717-313 - Rain can be found at the graveyard
448-844 - Don't jump at me - MXV
122-221 - Skunky!! - E.F.
004-400 - Watcha gun do ? - E.B.
550-550 - Go see Mortal Kombat the live tour!
282-282 - No Fear (it's a pinball game)
123-926 - No Knowledge that is not power
987-666 - Hold Flippers during casino run (pinball game reference)
Winner Fights:
969-141 - Winner Fights Motaro
769-342 - Winner Fights Noob Saibot
033-564 - Winner Fights Shao Kahn
205-205 - Winner Fights Human Smoke
2 on 2 only: (Only works in 2 on 2 mode)
227-227 - Explosive Kombat
022-220 - Explosive Kombat and Throwing Disabled