There is an unused Skull stage that can only be accessed by using Gameshark code 800FE55B 000A. At first the stage will appear black, but pausing and selecting "Restart this Match" will get it to appear properly.

Stage is largely a flat, circular arena with a unique texture painted on the ground. The skybox consists of an image of a skull stretched over the arena, with flames lit behind it. The stage also has some trouble loading the proper music from round to round.
Contributed by MeleeWaluigi
The unlockable character skin Meat got its name from a text posted on a website for Mortal Kombat IV's arcade promotion that read "Meat lives!" and the name was made official by later strategy guides. Despite being a mere alternate costume in Mortal Kombat IV, Meat made a canon appearance as a character in Mortal Kombat Deception and was eventually playable in Mortal Kombat Armageddon.
Contributed by Mass Distraction
It was originally intended for Kitana to appear as a fighter. Her color palette was swapped to yellow and thus created Tanya. While Kitana's model and textures were removed from the Playstation release, leaving only her moveset in the code, she is still fully functional in the N64 and PC releases, and can be accessed with a GameShark code or patch.
Contributed by KidDivinegon