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 Star Trek VI: The Undiscovered Country (1991), which was also a finale for its cast.
Robert Downey Jr. hid his own personal snacks all over set. The scenes where he's eating snacks and offers them to his costars were all improvised.
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.
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).
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).