- It takes a bit, but '70s Xavier eventually realizes who Logan is from First Class and gives him an Ironic Echo.
- Trask reads an excerpt from Xavier's thesis, which was written in First Class.
- Logan tells Xavier that he has fought in a lot of wars, something seen in the opening to Origins.
- Charles in 1973 regards Magneto as a monster, and Erik called himself "Frankenstein's Monster" in First Class.
- The sincerity of the younger Erik's apology ("I'm sorry, Charles, for what happened, I truly am") is questionable because it sounds very much like Shaw's from First Class ("I'm sorry for what happened in the camps, I truly am").
- When 1973 Xavier chooses not to inject himself with the serum (in order to regain his powers), a snippet of music from "Mutant and Proud" (from the First Class score) can be heard.
- When Xavier refers to his telepathy as, "This is a muscle I haven't stretched in a long time," it brings to mind the lesson he gave to Sean Cassidy in 1962. ("This is like any other muscle in the body, you can control it.")
- Stryker's "You hate mutants, don't you?" "No, I respect them" speech that he gives to the General in Origins is almost word for word repeated here, only this time a younger Stryker is on the receiving end as Trask (his mentor) delivers the speech.
- At the end of the movie, Logan is fished out of the water and placed on a grate much like how he was placed in X2 during his adamantium bonding process.
- Professor X's line where he informs Wolverine that the latter has to "Lead me, guide me" may have been recycled from the trailer for First Class where Charles tells Erik, "A new species is being born. Help me guide it, shape it, lead it."
- When Magneto recovers his helmet, The Rogue Cut adds a shot of him noticing the coin he had used to kill Shaw on display right next to it.
- In The Rogue Cut, a crippled war veteran asks Xavier how he became a paraplegic, and the latter's somber response is "Friendly fire."
- After a harsh discussion, Magneto tries to make peace with Charles... with a chess table.
- William Stryker is asked about his son, Jason, who played a crucial part in the events of X2.
- An exhibit in the Pentagon holds several mutant artifacts from the Cuban incident in the '60s: Sebastian Shaw's helmet, Angel Salvadore's severed wing, the coin used to kill Sebastian Shaw, and Havok's damaged focusing unit. Each of these were presumably recovered from the final battle in X-Men: First Class. The one exception is the helmet, which must have been obtained when Magneto was captured after JFK's assassination. Magneto had been wearing it when Azazel teleported the Brotherhood away from Cuba, and still had it on when he freed Emma Frost from government captivity at the end of the previous movie.
- Hank at first suspects that Peter is a teleporter because the teen's power reminds him of Azazel's.
- Kitty has a habit of insulting her enemies because she calls the Sentinel that is about to attack her an "asshole," which is reminiscent of the time she addressed the Juggernaut as "dickhead."
- This movie marks the first time Fassbender's Magneto addresses McAvoy's Xavier as "old friend," which is their elder counterparts' term of endearment in the original trilogy.
- While it's difficult to see, Colossus was sketching something before Warpath announces that the Sentinels have arrived. It's a small nod to X2: X-Men United where Colossus drew a funny image of Iceman being "zapped" by Rogue.
- While it's not explicitly stated, 2023 Logan is no longer the "ronin" we saw in The Wolverine because he has been reunited with his "master" Professor X, who gives him purpose as a warrior.
- In Xavier's study, the only book title that is clearly legible to the audience reads, "Einstein." As a boy, Charles used to decorate his night table with framed photos of his favourite scientists, including Albert Einstein.
- Numerous references to X2: X-Men United in the franchise include Xavier and Magneto uniting to prevent a potential mutant genocide, Magneto's non-metal prison (and guards with plastic weapons), weaponized metal balls, and turning a device created to destroy mutants against its human creators: New Cerebro in X2, the Sentinels here.
Continuity Nod / X-Men: Days of Future Past