While MCU fans forever lost three beloved superheroes in Avengers: Endgame, it was Tony Stark's Iron Man whose tragic death hit the hardest because not only was he a fan-favourite but also, his demise was something hardly anyone saw coming. That's why watching him die on-screen and Pepper telling him it's okay to let go left us with tears running down our face and a hanging jaw. But what we didn't notice was how Endgame retconned a scene from Infinity War to foreshadow Iron Man's death from the very beginning of the film. 

The clash of Iron Man and Thanos in Infinity War

In the 2018 film, Iron Man, Spider-man, Doctor Strange, and the Guardians of the Galaxy fought Thanos on the ravaged planet of Titan for the Infinity Stones. Despite their supreme efforts, he overpowered them all, even Iron Man who heroically tried to stop him single-handedly only for the mad Titan to seriously injure him. In the fight, Iron Man's suit was severely damaged and the left side of his helmet was destroyed. He started repairing his armor with his in-built nanobots but then redirected all his suit's powers to defeat Thanos and created the Katar, which only proved to be deadly for him. 

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We all know what happened next- Thanos reached Wakanda to take the Mind Stone from Vision and effectively ended half of the living beings on Earth. Back on Titan, everyone was dusted away except Tony and Nebula. Endgame begins with these two survivors adrift in space, almost out of all their supplies, and with no hope of rescue. Sensing that his fragile body, which had barely recovered after his fight on Titan, would succumb soon, he decided to record a message to Pepper through his helmet. 

Now, while we were busy wiping away tears after witnessing the emotional scene, a shrewd-eyed Redditor noticed a specific anomaly which we all seemed to have missed out on- interestingly, the helmet we see in the opening minutes of Endgame is damaged on the right side though we clearly saw that it was the left side that got destroyed in Infinity War. And as pointed out in the theory, this was a subtle change introduced by the filmmakers to foreshadow Iron Man's death later in the film where he was mortally wounded from his use of the Infinity Stones to beat Thanos. The damaged right half of the helmet seems to mimic his injuries at the end of the film. 

But there exists a logical explanation for this

The force with which Thanos hit Iron Man in Infinity War most probably completely shattered his helmet. After Tony was lost in space, he wanted to record a message for Pepper but as his suit was too damaged in the fight to be recovered, he used what was left of the nanobots to repair his helmet but didn't have enough to complete it, leaving the right side half-constructed. Though this was never shown or even hinted, it could be a plausible explanation. 

But even if this is what happened, for the filmmakers to choose to leave the right side of the helmet looking damaged is still a clever foreshadowing of how the Avengers fight against Thanos and his giant army would end with Tony's big sacrifice. You can check out the theory by redditor u/IronManMkXLVI here.