(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.
(at around 1h 45 mins) Edwin Jarvis, played by James D'Arcy, becomes the first character in the MCU to be introduced in a Marvel TV series and make an appearance in a movie. The character was first introduced in Now Is Not the End (2015) and was a regular cast member on Agente Carter da Marvel (2015). This also marks the first time the original Jarvis is seen in a film, as he had been replaced by the computer program J.A.R.V.I.S. (voiced by Paul Bettany) in previous films.
Stan Lee: (at around 1h 35 mins) his final cameo in a Marvel film. His '70s look based on his own actual look in the 1970s. They used Lola Visual Effects to de-age Lee back to his '70s self.
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 Capitão América: O Soldado Invernal (2014), Capitão América: Guerra Civil (2016), and in a deleted screen from Vingadores: Guerra Infinita (2018).