- The first shot in The Avengers is the Tesseract, an Infinity Stone central to that movie. The first shot here is the Chitauri scepter, which contains an Infinity Stone central to this movie.
- The film begins with the Avengers fighting HYDRA, who returned in Captain America: The Winter Soldier.
- After the abovementioned fight, we see a marble monument in New York honoring the first responders (but no Avengers) for The Battle of New York. At the end of the movie, the Creative Closing Credits gives the Avengers their own monument for the battle of Sokovia.
- Also from Winter Soldier — one of the personnel on Nick Fury's new Helicarrier is the same guy who refused to launch Project Insight, even with Crossbones pointing a gun at his head. Presumably, Fury picked him for his loyalty.
- Baron von Strucker is using Loki's scepter from The Avengers as the Plot Device in his evil plan.
- HYDRA has energy weapons similar to those used in Captain America: The First Avenger. These are indicated to be either reverse-engineered Chitauri technology, as he has one of the large transport creatures strung up in his lab, or the old HYDRA designs powered by the scepter instead of the Tesseract.
- Tony uses upgraded versions of his Iron Legion from Iron Man 3, which soon get hijacked by Ultron.
- When Hawkeye says "I've seen this before" in regards to people trying to lift the hammer of Thor, he's referring to how dozens of people tried to lift Thor's hammer and failed, even when they tried to pull it out with trucks.
- Hawkeye fights off Scarlet Witch's mind games and says "I've done the whole mind control thing. Not a fan," referring to his brainwashing by Loki in the first film
- At one point, Thor chokes Tony, like his brother Loki did in The Avengers.
- By the end of Hulk's rampage caused by Scarlet Witch's manipulation, Hulk lifts his head out of some rubble and gets knocked out by a punch to the side of the head from Tony in the Hulkbuster armor, mirroring what Hulk did to Thor in the previous film.
- When Maria is giving Steve her Expospeak on the Maximoff Twin's powers, followed by her simpler explanation, it's the same as the previous movie where Stark and Banner spoke science, leaving him confused.
- In The Avengers, Thor accidentally supercharges Iron Man's armour with his lighting, and later levels an entire forest by accident when he uses Mjölnir on Cap's shield. In this film, he and Cap use the latter technique deliberately to take out a HYDRA platoon, and Thor uses his lightning to supercharge the Vision's body and bring him to life. Amusingly, some of those things from the first film were flagged by critics for seeming like a Chekhov's Gun that didn't pay off.
- During the party scene, Sam mentions that he's doing work on their "missing persons" case while Steve is off being an Avenger, referring to the ongoing hunt for the former Winter Soldier, Bucky Barnes.
- Thor pours the Captain some Gargle Blaster that can leave a regular person in a stupor. Captain America: The First Avenger established that it's very hard for Steve to get drunk.
- The Red Room, where female spies and assassins like Black Widow were trained, should be familiar to viewers of the MCU-miniseries Agent Carter.
- Hulk, once again, one-shots the main villain with comically little effort. Twice in fact.
- The shot where Hulk spits out a tooth after being pummeled by Tony in his Hulkbuster armor is an almost exact recreation of Abomination doing the same thing in The Incredible Hulk.
- Once again, Bruce and Natasha have a Relationship Upgrade conversation in a child's bedroom.
- Wanda and Pietro's description of the unexploded shell that held them hostage is very similar to the one that mortally wounded Tony himself back in Iron Man. Except, of course, that Tony's detonated, and the Maximoffs' didn't.
- Remember in the first movie when Hawkeye was the one person perceptive enough to see that the portal could be opened from the other side? This time, he's the only Avenger who sees Witch's attack coming and successfully defends himself against it.
- Upon seeing Tony and Bruce trying to get Ultron's flesh-and-vibranium body running, Steve starts with "I'm only gonna say this once." Tony, who was drunk the last time someone said that, quickly responds with "How about none-ce?"
- An attractive blond who was shown to have an interest in the heroes in the deleted scene barely escapes death and stares out of the window while the villains attack.
- The first Avengers film played the song "Live to Rise" over the credits. This time, as he watches Sokovia fly, Ultron tells Stark, "You rise, only to fall."
Call Back / Avengers: Age of Ultron