Avengers: Endgame - 20 Things We Learned From The Director's Commentary

13. There's A Secret Screenwriter Cameo In 1970

When Tony and Steve Rogers (Chris Evans) travel to 1970 to retrieve the Space Stone and some more Pym Particles, Christopher Markus jokingly insists that the sequence contains "the best cameo in the movie."

And of course, keep your eyes peeled for the black suit-wearing G-Man who opens a door at the beginning of the sequence and you might notice that it's Markus himself, who refers to himself as "Mr. Chubby."

McFeely then makes him feel better by telling him that he "crushed it."