Other mistake: At the beginning, as Eggsy is pursued through city streets by the Golden Circle henchmen, the three pursuit vehicles are all firing rooftop mini-guns at Eggsy, in spite of the fact that they are all in single-file, one vehicle behind another. The Golden Circle vehicles would have been riddled with their own gunfire.Charles Austin Miller
Other mistake: One of the robot dogs spits a shell casing after being shot in the mouth. Casings are not propelled but discarded. It definitely made the sound of a casing and not a round so it may just be an audio error.