Barry quotes Leonard Snart's rules to making a plan: "Make the plan. Execute the plan. Expect the plan to go off the rails. Throw away the plan." He previously told Barry these in S3 episode 22 Infantino Street (2017).
When Oliver meets Felicity's Earth-X doppelganger, she has a yellow David's Star on her camp uniform. The word on the star spells Jood, which is Dutch for Jew. The German word would have been Jude. Dutch Jews had to wear the star on their everyday clothes as well, so they could easily be identified in public.
Paul Blackthorne, as "Sturmbannführer" Quentin Lance, uses an accent close to his native British English rather than the American accent he normally uses on Arrow (2012).
In the concentration camp scene, some of the prisoners were wearing the following identification markers: pink triangle, triangle with an inscribed letter p , and an orange triangle; which is a shorthand for homosexuals, poles (polish), and artists, respectively.
Felicity refers to Iris as "McClane." This is an allusion to John McClane, a character portrayed by Bruce Willis, in Die Hard (1988) and its sequels.