At the title screen, press A, B, A, Y, A, B, B (the Mortal Kombat Genesis blood code) to enable Big Head Mode.
Blue Screen of "Death"
As the game begins, before the title screen appears, enter the famous Konami Code Up, Up, Down, Down, Left, Right, Left, Right, B, A before title screen.. You'll encounter an "error" screen for laughs. Press any button to return to normal gameplay.