While being interrogated at GCPD, Zsasz says he "would've covered his entire body in sweet, sweet scars." This is a reference to the comics, wherein he is a deranged serial killer, and cuts "tally marks" on himself for each of his victims. His scars were previously seen in Penguin's Umbrella (2014).
Jeremiah Valeska's hat highly resembles one of the hats Joker wears in The Killing Joke graphic novel.
Nygma's Riddle is: "I was there from your first breath, and I will follow you until your death." The answer is: a name.
The statue that the Old Lady uses to hit Nygma resembles the Maltese Falcon.
The production code was T40.10054.