Continuity mistake: When Spidey takes Cap's shield he uses his web on it, but when he lands on the car the web on the shield is gone.
Continuity mistake: After Falcon's drone shoots the man with the vial, it begins to fall to the ground and Black Widow catches it. In the close-up shot, the lid is in the palm of her hand, but when she gets up the vial has turned around, with the lid now above her hand. (00:11:00)
Revealing mistake: During the airport scene, after Tony tells Wanda that she hurt Vision's feelings, Tony and Hawkeye have their own banter moment. Then, when Hawkeye says, "Made you look," the next shot as the car is falling, Hawkeye is reversed; note his arm guard. (01:32:15)
Trivia: Stan Lee makes a cameo at the end of the movie as the FedEx man who says "Are you Tony Stank?"
Trivia: After Tony blows off Bucky's arm, Cap picks up his shield and shields himself from Tony's blasts. At the very end of the shot, Tony is firing a beam at Cap which lands on Cap's shield and the shot begins to slow down. This moment from the shot is an homage to the cover from part 7 of the Marvel Civil War comic storyline, where Captain America and Iron Man are fighting on the front cover in the same way. (02:07:00)
Chosen answer: We never get a specific explanation to what happens to him when he enlarges. Hank Pym explains to him that when he shrinks, it's the result of reducing the distance between atoms which is why he retains his original mass. However, being a giant with a mass and strength of only 160lbs would not be very advantageous in a fight, so they must have found a way for him to grow in size and strength in this situation. Ultimately, we just don't know - the movies and comics differ in a great many respects, so there's just not enough information to work with. Might technically qualify as a mistake, but it's something which could easy be solved with an in-universe answer, however scientifically dubious.
Phaneron ★