Two of the founding Avengers, Iron Man and Captain America, have been often at odds with each other, but the one act that Steve Rogers could never forgive was Tony Stark's biggest lie. While struggling to find a way to stop the Incursions, apocalyptic events that destroyed the entire Multiverse, Iron Man made Captain America and the other heroes believe they still had a chance to prevent the end of the world, when there was actually none.
Ever since Civil War, the relationship between Captain America and Iron Man has been conflictual, to say the least. While the two heroes eventually overcame their disagreement over the Registration Act (even if it took the death of Cap to do that), another crisis erupted when Steve found out about the Illuminati, a secret cabal created by Tony and the most powerful heroes of Earth to solve the biggest problems, with no accountability and little concern for the methods used. When Captain America discovered the group, his trust in Iron Man received another blow, but he decided to become a member of the Illuminati too, until the Incursions began and Steve's opposition to the use of weapons of mass destruction to save Earth-616 resulted in his memory being erased. When Captain America later recovered his memories, he mercilessly hunted down the Illuminati and Iron Man.
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When the final Incursion occurred, the entire Multiverse was just a few moments away from total annihilation. In Avengers #44, by Jonathan Hickman, Stefano Caselli, Kev Walker, and Frank Martin, Steve finally confronts Tony to hold him responsible for all his lies and betrayals. What hurts Steve most is that, after the Illuminati and the rest of the heroes decided to work together to find a solution to the Incursions, Iron Man gave everyone false hope, when he knew that there was no solution possible. This is a heartbreaking fight, as it doesn't end in reconciliation - Captain America and Iron Man die in a battle with each other, only to be "reborn" with the next iteration of the Multiverse after Secret Wars.
What makes Tony's lies even more tragic is that, as revealed at the beginning of the issue, even Captain Universe, the embodiment of the universe itself, is angry at him for giving everyone false hope. This was a crime against Steve - who spent his last days fighting the inevitable and making heartbreaking choices - but also a crime against the universe itself. Captain America and every other hero on Earth did unspeakable things to protect the planet, when its destruction would have saved the rest of the universe, thinking that their sacrifices were all done to buy time for a final solution that Iron Man and the other "geniuses" would find at some point. But it was all a lie.
Steve was so angered by this that he decides to spend his last hours alive gearing up in a battle suit and fighting Tony, rather than making peace with himself, as could be expected by someone with Cap's history and personality. Iron Man's sins, perfectly explained by his final insult that, coming back, he would do everything in the same way, were so unforgivable that they broke Captain America, turning him from the wise old soldier into a man full of rage who spends his last moments on Earth punching his former best friend in the face.