WARNING: The following article contains spoilers for Avengers: Endgame, in theaters now.
Avengers: Endgame revolves around Earth's Mightiest Heroes building a time machine to steal the Infinity Stones throughout various points in history, all so they can return to the present and bring the "dusted" back.
It's no surprise that, with time travel in play, not to mention a three-hour runtime that virtually brings in every hero we've seen in the Marvel Cinematic Universe thus far, a few plot holes arise. Let's break down some of the more obvious ones.
THE TIME HEIST
When Hulk meets the Ancient One in 2012's Battle of New York, the plan shifts a bit, which results in the Avengers borrowing the Infinity Stones on the condition they return them to their timelines at the same moment they were taken. This is the only way to leave the timestream unaffected and not create alternate realities.
However, on multiple missions, people constantly break the rules and tinker with the past. In 2012, Steve Rogers fights himself right after Loki escapes with the Tesseract, not to mention he changes Hydra's actions in Avengers: Age of Ultron when he pretends to be one of them. Steve also chooses to remain in the past with Peggy Carter in the finale, which affects how the Avengers initiative kicked off, as well as the wars they fought after. This ripple would have affected everything downstream, including the snap.
THE VORMIR SACRIFICE
Just before the Avengers go on their temporal mission, they divide into teams and do a breakdown of where to find all the Infinity Stones. They detail how to extract them, because they know they shouldn't do anything to alter the timestream. Yet, when they get to Vormir, Hawkeye and Black Widow somehow don't know they need a sacrifice for the Soul Stone.
It's a pretty dumb moment because Nebula knew Thanos killed Gamora to obtain it in Infinity War, so she should have given the duo a heads up. It appears Nebula forgot to tell them for the sake of plot convenience, all so the Avengers could dramatically duke it out to see who draws the short straw.
TEAM THANOS' TRIP TO THE FUTURE
The first plot hole here revolves around evil Nebula infiltrating the Avengers in the present. When the Avengers snap the "dusted" back to life, evil Nebula uses the time machine to open a portal allowing Thanos' legion to come to the future. However, the quantum tunnel Tony Stark created doesn't work like that: He made it clear the only objects they could pull from the timestream must have the mysterious wrist devices, which are basically temporal GPS locators or tags for his time machine to pull.
The second plot hole arises in the finale, when Tony uses his tech gauntlet to snap Thanos' army away. Killing them here means they don't exist in the 2014 timeline anymore, thus nullifying everything from Guardians of the Galaxy onwards. Once more, without Thanos and his forces there, history's changed drastically and the snap wouldn't ever happen. The main timeline is further affected when Gamora is also taken from 2014 and doesn't return to her timeline.
NEBULA'S PARADOXICAL DEATH
When evil Nebula tries to steal the Stark Gauntlet for the Mad Titan, she's confronted by the redeemed Nebula and Gamora in the present. However, in a very emotional standoff, the good Nebula shoots her past self in the chest, killing her.
It's a basic time travel rule being broken here, because by killing her past self, future Nebula shouldn't exist. Yet, once again for plot convenience, future Nebula's alive and well and goes on to help take down the rest of Thanos' troops. Also, killing Nebula from 2014 affects everything that leads to the snap, thus adding to the series of paradoxes, wherein the very thing Endgame is based on wouldn't happen at all.
THANOS USING THE STARK GAUNTLET
In the finale, after winning a brawl with Captain Marvel, Thanos puts on the Stark Gauntlet, but the Russo brothers apparently forgot the rules they created for this. This gauntlet isn't designed to fit the Mad Titan, it's nanotech is programmed to fit Avengers only. That's why the Hulk wields it to snap the deceased back to life and also why Tony dons it to snap away Thanos' forces. Thanos shouldn't be able to force his hand into it or else it'd break
Another plot hole arises when Thanos wears it and snaps his fingers, not realizing the stones are missing. Upon rewatching the scene, Thanos looks at the gauntlet before he snaps, so he should have seen the stones absent. More importantly, his mind and body are connected to them, so once Stark took them back, the Mad Titan should have felt the link being severed and knew he couldn't perform another snap.
Directed by Joe and Anthony Russo, Avengers: Endgame stars Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Mark Ruffalo as Bruce Banner, Chris Hemsworth as Thor, Scarlett Johansson as Black Widow, Jeremy Renner as Hawkeye, Brie Larson as Captain Marvel, Paul Rudd as Ant-Man, Don Cheadle as War Machine, Karen Gillan as Nebula, Danai Gurira as Okoye and Bradley Cooper as Rocket, with Gwyneth Paltrow Pepper Potts, Jon Favreau as Happy Hogan, Benedict Wong as Wong, Tessa Thompson as Valkyrie and Josh Brolin as Thanos. The film is in theaters now.