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.
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)
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