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The official debut of Ermac is in Ultimate Mortal Kombat 3.
But he has been in the series since the first episode as a glitch.
The fact is the actor playing Sub-Zero, Scorpion, Reptile and Smoke was wearing a red costume whose palette was digitally changed into blue, yellow, green and grey. But occasionally this wasn't happening, and so the character was appearing in the original red color and named Ermac, that stands for ERror MACron. The fans belived for years of having discovered a secret character, while they had just found a bug in the game. However, due to his increasing popularity, Midway decided to make him an official character.

Source:

http://mortalkombat.wikia.com/wiki/Ermac

http://it.wikipedia.org/wiki/Ermac

Oh come on, this deserves to be one :/