- Acting for Two:
- Stephen Amell is playing both Green Arrow and his Earth-X counterpart Dark Arrow.
- Melissa Benoist acts as both Supergirl and her Earth-X counterpart Overgirl.
- Tom Cavanagh is both Harry Wells and Eobard Thawne.
- Ascended Fanon: Of a sort. Before The Reveal that Prometheus was actually Adrian Chase, a very popular theory was that it was Tommy Merlyn, having either faked his death or been brought Back from the Dead. Indeed, on Earth-X, Prometheus is Tommy Merlyn.
- Ascended Meme: Oliver and Kara, or SuperArrow, became the most popular Crack Ship people jokingly supported over Olicity. Their Earth-X counterparts are a married couple which, due to a combination of the genuine chemistry between the two of them, the repaired relationship between their Earth-1 counterparts, and the now infamous double wedding, caused the popularity of the ship to skyrocket after this crossover was over.
- Chronically Killed Actor: Colin Donnell continues his streak of playing characters that get themselves killed.
- Deleted Scene: In the fourth episode, there was a scene where Harry meets Thawne. Thawne offers Harry a place in the New Reich because of his intelligence, to which Harry of course refuses. Thawne then tries to kill him via his vibrating hand only for it to be repelled; after his encounters with Zoom and Savitar, Harry injected his body with millions of nanobots programmed to attack any foreign cells trying to enter his system, after which Thawne quickly leaves.
- Dueling Works: With Justice League (2017), DC's other big Avengers Assemble event of 2017, which premiered two weeks before the crossover between the CW shows. Even with the controversial elements of the crossover (such as the emphasis on relationship drama in Part 2 and the despised ending), most generally agree that the crossover overall was better than Justice League.
- Executive Meddling: Apparently it was Greg Berlanti's idea to end the crossover with the now infamous double wedding. Given what happened in the subsequent "The Flash" episode it seems like the writers would've rather not ended it that way.
- Fake American: British Russell Tovey portrays The Ray, with a southern twang accent.
- I Knew It!:
- Many fans were right on the mark that Eobard Thawne managed to worm his way back from non-existence yet again, appearing on Earth-X as the Dark Flash.
- People theorized that either Tommy Merlyn or Roy Harper was Earth-X Prometheus, since Colin Donnell was reported to be seen on set and Colton Haynes tweeted he would be returning to Arrow. It turned out he was the former.
- Ever since Victor Garber announced he was planning to go back to Broadway, it was teased that Martin Stein will be retiring. And because of Retirony, many people correctly guessed that he would die in the crossover instead of actual retirement.
- Queer Character, Queer Actor: Coincidentally or not, Russell Tovey and Wentworth Miller who play the Ray and Citizen Cold (respectively) are gay in real life too.
- Reality Subtext: Just three months before this crossover aired, Neo-Nazis marched in Charlottesville, VA and caused a large amount of riots in the city, leading to the death of a woman caught in the crossfire and the nationwide realization of how widespread the Nazi support is in the United States. This crossover showcasing these same fascists as unambiguously evil couldn't have come at a more prudent time. What's more a popular Supergirl cosplayer Alice Kiss took part in the protests, shortly before the character of Overgirl was introduced into the Arrowverse.
- Real Life Writes the Plot: Katie Cassidy was originally slated to be part of the crossover, but she had to take time off due to the recent death of her father. As a result, Siren-X's appearance was pushed back to later on in The Flash with the Hand Wave that she was left in charge of the planet while the rest of the team was gone, and she's pissed at Earth-1 for killing her ex-lover Oliver.
- Throw It In:
- Caity Lotz improvised the "How's your butt?" bit, much to Chyler Leigh's genuine surprise.
- Leigh improvised the "sassy assassin" bit during the conversation Alex and Kara had right afterwards.
- Troubled Production: A more mild example than most, but by all accounts this was not an easy event to produce: trying to coordinate the event around each show's schedule was a nightmare; this is why the Legends were left out of the next crossover Elseworlds and they weren't meaningfully involved on Crisis on Infinite Earths (2019).
- What Could Have Been:
- Katie Cassidy was supposed to show up in some capacity, but then her father died so her planned appearance was written out so she could go to his funeral. The Flash episode, "Fury Rogue" revealed she would've been cast as Siren X, the New Reichsmen's answer to the Black Canary. Siren X makes a later appearance; she was Oliver-X's ex-lover and pissed that the Earth-1 heroes killed him.
- Marc Guggenheim tried to get this released as a single movie instead of a crossover spread out over four shows, including new material. But "union rules regarding credit" put on kibosh on the plans.
Trivia / Crisis on Earth-X