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)
Revealing mistake: During the face-off between Bucky, Captain America and Iron Man at the end of the film the fake snow is seen stuck to the actors after they fall down. It remains throughout the scene rather than melting as real snow would.
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 ★