The music that can be heard playing from Cassie Cage's earphones is the music from the Mortal Kombat Trilogy options menu.
Ermac is an abbreviation for the term "Error Macro." His existence came about in Mortal Kombat 1 when a glitch occurred that rendered Scorpion's costume red. Mortal Kombat creator Ed Boon then added Ermac into the final game of the original trilogy.
Quan Chi can be seen in some some of the fight intros holding the severed head of an Oni. This is actually the character Moloch, who debuted as a sub-boss in Mortal Kombat: Deadly Alliance (2002). His only other appearances were in Mortal Kombat: Deception (2004), in which he made a brief appearance in Konquest mode, and in Mortal Kombat: Armageddon (2006), which was an "every character" game.
First Mortal Kombat game to feature brutalities since Mortal Kombat: Shaolin Monks (2005). Brutalities were previously featured in 1996's Mortal Kombat Trilogy (1996) and the console versions of Ultimate Mortal Kombat 3 (1995). Mortal Kombat vs. DC Universe (2008) also introduced "heroic brutalities" for its DC characters who did not kill their opponents.
Leatherface's inclusion in the game marks the first time the character has been in a video game since 1983.