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 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: In 2012, when Tony flies up inside the Stark Tower snooping on the Avengers talking to Loki, Loki delivers his lines faster compared to the original Avengers movie, and his movement is different too. (01:16:15)
Stupidity: They make a big deal about how they've got the bare minimum of Pym particles left as Hank has been snapped away, but it never occurs to them to use their supply to jump back to a very safe time when Hank was around with a vast supply of them. They could bring them to their time and then have no end of attempts to get the stones without being on such a knife-edge.
Character mistake: When they infiltrate the military facility, Steve and Tony are trying to blend in. Steve wears his hat the entire time he's indoors. He's a soldier, taking his hat off when he comes through the door would be second nature to him. Not removing it would make him stand out as someone who doesn't belong.
Other mistake: The original Avengers movie showed that the Chitauri can be disabled when their control ship is destroyed. In Endgame, the only possible control ship is Thanos' ship, but we still see Chitauri on the battlefield after Captain Marvel destroys the ship.