Continuity mistake: In the final battle, Wasp and Ant-Man are in the van trying to get the quantum tunnel operational. We cut back to the fight and we can see Ant-Man there too, fighting in his giant form. (02:22:20 - 02:23:00)
Continuity mistake: When Thanos is being questioned about where the stones are, Captain Marvel has her arm wrapped around his neck in a headlock and War Machine is holding onto his right arm. Just as Bruce goes to push Thanos over, both Captain Marvel and War Machine suddenly do not have their hands on him. There is no reason that they would have let go of him. Nor would there have been enough time for them to let go of him in between shots. (00:18:10)
Continuity mistake: In the first Avengers film Steve lost his helmet after the battle and got dirty, with a damaged suit and battered face, but here while they are arresting Loki he has the helmet on and the suit is intact and clean. Can't be that he cleaned himself up, because in the Avengers post-credit scene eating shawarma (after arresting Loki), Cap's suit is still dirty from the battle. Can't be that he got changed before heading down (and why would he?) because in the shawarma scene from the first Avengers movie he's still in his damaged/dirty suit after everything.
Continuity mistake: When the camera is panning over the entire army on the Avengers' side for the first time, Captain America is standing in front of everyone, and his shield is intact and round again, while Thanos broke it earlier. The next time we see it, it's back to being broken.
Continuity mistake: When the Avengers return from the quantum tunnel with the stones, their order is Hulk, Tony, Steve, Clint. But seconds later this changes to Hulk, Steve, Clint then Tony. (01:55:18)
Continuity mistake: At the end of the movie, when Thor is speaking with Valkyrie about being the new Queen of Asgard, his beard is trimmed and much shorter. In the next scene aboard the ship with the Guardians when he and Quill are arguing about who is in charge, Thor's beard is back to how long it was earlier in the film. (02:40:20 - 02:41:45)
Continuity mistake: When the Avengers are discussing where Thanos might be, Steve tells Tony that he fought him. In the shot where Tony says "Who told you that", his right hand is holding onto the robe he is wearing. The camera changes angles and his right hand is now shown touching the table. It then cuts back to Tony, where his right hand is touching the robe again. (00:10:39)
Continuity mistake: When Nat reunites with Barton in Tokyo to tell him about the plan, whilst he says "Don't give me hope", there are a few wet strands of hair shown going over his fringe. It then cuts to Nat, then back to Barton. In the second shot of Barton, the hair across his fringe is suddenly a lot more messier. (00:57:15)
Continuity mistake: When the Ancient One is showing Bruce the flow of time, a projection of the Infinity Stones are shown rotating around the flow of time she projects. Just as Bruce says "Because once we're done with the stones...", the projection of the Space Stone is shown rotating away from Bruce and more towards the Ancient One. The camera changes angles, and the Space Stone is now closer to Bruce, and is shown rotating past the same area it did in the previous shot. (01:24:30)
Continuity mistake: In the Hulk scene at the diner, once Scott takes the picture of the kids, in the frontal shots he is holding the phone with his right hand and hands it to them, while the reverse shots don't match - egregiously noticeable when he says "I don't want a picture with them" waving an empty right hand. (00:38:05)
Continuity mistake: When Tony takes off the Nano Arc Reactor and places it on Steve's hand, he drops to the ground. Captain Marvel is shown in the back of shot, raising her hands slightly as if she is preparing to catch him. It then cuts to a wide shot where her hands are suddenly at ease. (00:12:15)
Continuity mistake: Iron Man, Cap, Ant-Man and Hulk are shown leaping into 2012 just as the original six Avengers formed their circle. Cap then immediately begins reminding the team of their assignments. The dialogue is continuous so there is no time compression involved. Within 10 seconds of their arrival, they watch 2012 Hulk jump into a nearby street and begins smashing Chitauri and jumping on a car. However, in the original film, after the Avengers form the circle, the events that follow include Loki ordering more troops, then Cap giving orders to the rest of the team to hold them off, ending with the order for Hulk to "smash." Hulk then jumps up and starts fighting Chitauri along the sides of buildings before being knocked down into the street, where he could have presumably landed where the 2023 team sees him (the original film cuts to Thor at this point). These events in the original film take at least a minute to unfold in real time. (01:07:35)