Following Thanos' victory in Avengers: Infinity War, it's a pretty good bet Earth's Mightiest Heroes and the surviving Guardians of the Galaxy will resort to desperate measures in order to defeat him. Speculation has it the surviving heroes will actually try to undo the Mad Titan's actions via some sort of time-travel, and while the Rosso Bros. certainly aren't speaking, numerous set photos seem to back this train of though up.
One particular Endgame theorist is pretty confident the Avengers will succeed in this, purporting that evidence already exists of their success in the very first Infinity War trailer. According to Reddit user BigMartin58's hypothesis, what we thought was a straightforward teaser for Infinity War may have actually been the first trailer for Avengers: Endgame -- sort of.
When Infinity War came out, several scenes didn't make the final cut. The Russo brothers said some sequences were inserted into the film's marketing in order to throw the audience off, while others were simply edited out in post-production to serve the narrative. But if time travel does play a central role in Endgame, it stands to reason that the past could be altered to something that resembles at least some of Infinity War's cut or altered scenes.
The NYC Showdown
The first change that comes into question in said trailer is at the 1:00 mark, where we see Robert Downey Jr.'s Tony Stark taking off his shades with his colleagues in the background. This is during the battle of New York where Ebony Maw and Cull Obsidian come to claim the Time Stone from Doctor Strange.
In the movie, by the time Stark removes his glasses his armor is already forming around his body thanks to his nanoparticle technology. The trailer, though, has him still in his black top before he armors up. Could this be from an Endgame scene where Stark feels he doesn't have to rush to Iron Man-out ASAP? It's unlikely, because even if something prevents Cull Obsidian from showing up, the Maw's telekinesis alone would still require everyone to defeat him. Seeing as he already showed he could take down the Sorcerer Supreme, there's no reason for Stark to go unprotected agains him, or anyone Thanos sends on the whole. Odds are this is a case where the final effects weren't finished by the time the trailer hit.