(at around 41 mins) Robert Downey Jr. once told Anthony Russo and Joe Russo about how one of his children said "I love you 3000" to him. The directors liked the phrase so much that they decided to include it in the film.
When Avengers: Endgame (2019) passed Titanic (1997)'s box office total, James Cameron sent a congratulatory message to Kevin Feige and Marvel Studios on dethroning his film - with a photo of the Avengers 'A' being the iceberg that sinks the Titanic.
The idea of showing the autographs for the original The Avengers (2012) actors in the end credits came from Rotta verso l'ignoto (1991), which was also a finale for its cast.
Jim Starlin: (at around 21 mins) the creator of Thanos appears as a member of the support group. He is the bald man with the glasses and a goatee.
Ken Jeong: (at around 23 mins) as a security officer when Scott Lang returns to his van in a storage locker. The Russo brothers previously worked with Jeong on Community (2009).
Yvette Nicole Brown: Woman in the elevator at the army base who asks Cap, "Are you new here?" The Russo brothers previously worked with Brown on Community (2009).
Matthew Berry: (at around 1h 20 mins) ESPN's fantasy football expert portrays one of the S.H.I.E.L.D. agents alongside Alexander Pierce (Robert Redford) during the confrontation with Tony Stark in the New York scene.
Joe Russo: (at around 20 mins) as the support group member who talks about going on a date for the first time after losing his male partner, marking the first time an openly gay character has been featured in the MCU. He earlier made cameos in Captain America: The Winter Soldier (2014), Captain America: Civil War (2016), and in a deleted screen from Avengers: Infinity War (2018).