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)
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)
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: Because the U.N. has been preparing to pass the Sokovia Accords and they want to oversee the Avengers. A week earlier, the Avengers would not have had to seek permission to find the other Winter Soldiers (or wait while the oversight committee makes a decision). As a result of the U.N. Sanctions, Captain America and Sam Wilson have to go rogue in order to find the other soldiers. (Their actions of going rogue result in Iron Man and his team in hunting the rogue Avengers down, creating a "civil war").
Bishop73