Psycho Cyan Bug
From Final Fantasy Wiki
The Psycho Cyan bug is a bug in the game Final Fantasy VI. It is actually the combination of two bugs involving Sky. The first bug allows Cyan to counter any physical attack dealt by any target if he is KO'd and revived while in "Sky" status. The second bug occurs when Cyan is Imped while in "Sky" status. He counterattacks with a simple physical attack instead of Sky itself while Imped, and as such, "Sky" status never comes off. When these two bugs are combined, Cyan enters an infinite loop of attacking his opponents. Because he is Imped, "Sky" status never comes off, and because he was revived after being KO'd, he effectively attacks, and counter-attacks his own attack with another attack. The loop ends when all enemies on the screen are dead.
This bug was "fixed" in Final Fantasy VI Advance. However, it can be obtained once certain requirements are met. First Cyan has to die. After that is accomplished, revive him and do Sky. Imp him at this point and he will go "Psycho Cyan."
