Crisis on Earth-X, Part 3 - S4-E8
Trivia: Supergirl says to her doppelganger "General, can you step outside." That line was said by Superman in Superman II.
Going Rogue - S1-E4
Trivia: Kahndaq is mentioned heavily in this episode. Kahndaq is a fictional country, home to DC anti-hero, Black Adam.
Attack on Gorilla City - S3-E13
Trivia: Julian asks Barry "are you gonna go to the planet of the apes?" Tom Felton played Dodge Landon in the modern remake of that film.
Trivia: Robbie Amell, who plays Caitlin's fiancee Ronnie, is the cousin of Stephen Amell, who plays the lead role on Arrow.
Trivia: When The Atom first appears, the characters say "It's a bird" "It's a plane", an obvious reference to Superman, played by Brandon Routh in "Superman Returns", who also plays The Atom in this episode.
Tricksters - S1-E17
Trivia: Mark Hamill plays James Jesse, the original Trickster, a role he also played twenty-five years earlier in the 1990 Flash TV series.
Things You Can't Outrun - S1-E3
Trivia: The movie Barry and Iris are leaving is entitled Blue Devil II. A reference to DC Comics superhero Blue Devil, who was a stuntman that worked on the in-universe "Blue Devil" films.
Trivia: Cisco plays the Queen song "Flash" while Barry is running through the black hole. This song is actually a reference to Flash Gordon (used as the theme song for the 1980 film), not the speedster named Flash.
Trivia: There is a reference to Smallville TV show in the opening scene among the heroes - Green Arrow played by Justin Hartley is lying on the ground.
Trivia: The music made by Danny Elfman for Batman and The Flash movies can be heard in Gotham city.
Trivia: Elizabeth Tulloch, who plays Lois Lane, is married to David Giuntoli, with whom she starred on Grimm, and who had auditioned to play Clark/Superman in Man of Steel.
Trivia: Sherloque develops a crush on Renee Adler, a clear play on "Irene Adler" from other Sherlock Holmes media. Although only appearing in one book, in TV shows/movies Irene is frequently a love interest of Holmes.
Trivia: When Barry is suffering amnesia he asks to be named Bart, Bart Allen in the comics is grandson of Barry and the future Flash.