After watching Avengers Endgame, I have drawn some conclusions. I noticed some mistakes. I am going to list them along with the possible explanations for those mistakes.
However, first thing before I list those mistakes, I want to warn you that this article contains spoilers. If you have not seen Avengers Endgame, then please do not read beyond this point.
Second thing that I want to point out that Marvel Comics implement the concept of time travel in a different way compared to DC Comics and all other comics. In all stories related to time travel, the change in a past event leads to change in future. The Flashpoint story arc of DC Comics and TV shows like DC‘s Legends Of Tomorrow follow this phenomenon. However, in Marvel Universe, attempt to change the history creates an alternate reality in another dimension without making any impact in the world where the attempt has been made. Maybe this serves as an explanation for timeline mess but this explanation is also flawed.
Coming to mistakes:
1. Black Widow and Hawkeye went in past before Thanos got soul stone. Black Widow sacrificed herself. Hawkeye got the soul stone. Now Gamora should be alive as Thanos cannot sacrifice her to get soul stone.
2. In the climax, Captain America decides to settle in 1940s, marry Peggy Carter and grow old with her. Falcon mentions growing up in a world without Captain America. This should change events of Captain America: The Winter Soldier, Captain America: Civil War, and Avengers: Infinity War. However, Falcon remembers his adventures with Captain America despite mentioning that Captain America‘s decision to settle in the time period of 1940s made him miss the opportunity to work with Captain America, and he lived in a world without Captain America.
3. In the climax, Iron Man kills Thanos of 2014. If Thanos of 2014 dies, he never becomes the Thanos of 2018 who killed half the universe in Avengers: Infinity War.
4. If Infinity War does not happen, then Avengers: Endgame should also never happen.
5. Many timeline changes happen due to superheroes collecting Infinity Stones. Thor getting his hammer Mjolnir in 2013 can disturb the events of Thor: The Dark World. However, those changes can be ignored, assuming that in the climax, superheroes had returned the stones and all objects into that moment of time from which they had taken them.
6. Thor kills Thanos of 2018 three weeks after the events of Infinity War. After five years, superheroes scheme against Thanos starts. This has been established by showing Tony Stark (Iron Man) having 5-years old daughter. And Ant-Man‘s daughter appearing as a young girl in her late teens. This means that the final battle between Avengers and Thanos of 2023 will take place in 2023. Nebula of 2023 kills the Nebula of 2014 and continues to stay alive.
7. Captain America appears as an old and aged man in the climax. How did he age? And why Winter Soldier did not age?
8. In the climax, Falcon is talking to Winter Soldier as if he remembers what happened during the events of Captain America: Winter Soldier movie. But how? With Captain America settling in 1940s, the events of Captain America: Winter Soldier can never happen the way they happened.
9. With Infinity War never have happened, Ant-Man should never have got stuck in quantum realm.
If I find more mistakes after finding deleted scenes from this movie, I will add them in this blog post. Coming back to the possible explanation, attempt to change the history in the Marvel universe creates an alternate reality in another dimension without making any impact in the world where the attempt has been made. However, if that was true, then the events of Winter Soldier movie and Civil War movie would have happened differently due to Captain America settling in 1940s and Falcon mentioning that he lived all his life without Captain America. So Avengers: Endgame was an entertaining movie but not logical, given how it contradicted its own theories.