In a scene near the end, Curtis is using a computer/device in Ollie's lab with a 2001: A Space Odyssey (1968)-style label on it reading "HAwryLiw 3000", a call-out to the "HAL 9000" computer in A Space Odyssey. Ken Hawryliw is the series property master.
While not having as many as the previous episodes, this episode has some callbacks and references to previous seasons. Slade's attack on Star City and the events from episode "City of Blood" to "Unthinkable" is referenced multiple times during this episode. Curtis jokingly names Grant's attack "Siege 2.0", but instead of Mirakuru soldiers, it's bombs. Grant mentions how his father attacked Star City 6 years ago and how he is planning to the same. When the criminal in SCPD takes a seat, he asks an officer what the time is, then puts a bomb under the seat, puts on a Deathstroke mask, and begins taking out cops. A very similar thing happens during "City of Blood", where it is similarly the start of a siege. Diggle mentions that they need a new strategy, instead of going to the tunnels to fight Deathstroke's army, as they did in "Unthinkable". After Mia defeats Deathstroke, Oliver uses a rope arrow to tie him to a wall, similarly to how he did in "Unthinkable". During Rene's speech, Laurel prompts Dinah to try dipping fries into her Big Belly Burger milkshake, which Laurel (Earth-1) also told Nyssa to do in "Al Sah-him". Diggle talks about his failure on Andy as he was killed by John in "Genesis". Connor's knowledge and hatred towards Grant could be reference to "Star City 2046", as both Joseph David-Jones and Jamie Andrew Cutler guest-starred in it as Connor Hawke and Grant Wilson respectively. In that episode, Grant was also known as Deathstroke and leader of his own Deathstroke army and Star City.
Laurel mentions to Dinah the butterfly effect. In chaos theory, the butterfly effect is the sensitive dependence on initial conditions in which a small change in one state of a deterministic nonlinear system can result in large differences in a later state. The idea that the death of one butterfly could eventually have a far-reaching ripple effect on subsequent historical events made its earliest known appearance in "A Sound of Thunder," a 1952 short story by Ray Bradbury. "A Sound of Thunder" discussed the probability of time travel.