The "Guardians of the Galaxy" actor shared the sneaky clip on Instagram early on Monday, April 29, admitting he didn't want to miss the chance to document the movie moment early on in the Disney/Marvel superhero blockbuster's production.
The clip features Chris quietly planning the camera around the room to show the likes of Dave Bautista, Chris Evans, Robert Downey Jr., Mark Ruffalo, Danai Gurira, Chris Hemsworth, Jeremy Renner, Chadwick Boseman, Paul Rudd, Elizabeth Olsen, and Don Cheadle talking through a scene with filmmakers, with some of the stars in costume, and others in casual clothing.
Pratt then turns the camera on himself and confesses, "This is a really illegal video."
"Captain America" star Evans then walks up to Pratt and jokes he's in "big trouble, big time" as he throws a playful punch at his co-star.
In the accompanying caption, Pratt explained why he broke the rules on the set of "Avengers: Endgame", which closes out the current phase of the franchise.
"I remember being blown away in this moment on the @avengers set," he wrote.
"Nobody was allowed to film anything on their phones. I said screw it. No rule was going to stop me from seizing this once in a lifetime opportunity to capture this collection of stars, a groupthat (sic) likely will never be in the same room again. We are so blessed."
Pratt's Instagram post emerged as he and the Endgame cast and crew celebrated the film's monster box office opening, breaking records worldwide by debuting at number one with a global gross of $1.2 billion.
He wasn't the only one to mark the special occasion with old footage on social media - Gwyneth Paltrow, who has played Pepper Potts since 2008's "Iron Man", the first in the Marvel Cinematic Universe series, reminisced about the franchise in her own post on Instagram on Sunday.
"What a weekend! From Ironman 2 with this crew all the way to behind the scenes of #avengersendgame ...amazing to be a part of such an impactful franchise," she captioned a photo of herself with Downey, Jr., Cheadle, and Scarlett Johansson.
"Congratulations to the giant team that made this film (Endgame) possible, starting with ('Iron Man' director) @jonfavreau it all started with you."