Continuity mistake: At the start, the way Harry holds the medal of valor changes between shots. (00:03:20)
Continuity mistake: When Eggsy is in the interview room at the police station, in a wide shot the tape recorder is on the table by itself, with nothing around it. Eggsy demands a phonecall, the police officer leaves, and Eggsy then reaches beside the recorder and picks up a cordless phone which has appeared from nowhere. (00:16:45)
Continuity mistake: When Eggsy meets fellow cadets and one of them asks if he's served him in McDonald's, in one shot, Eggsy folds his arm before replying, and then in another, does the same again, from a different angle, while replying. (00:31:45)
Other mistake: In the "Uncut" version of the film, after the "Dinner Party" at Valentine's place, when Valentine is at the door saying goodbye to Harry, he says "Just give me a couple of days to think over your proposal." His lisp has disappeared on the word "days." (00:57:00)
Factual error: When the barracks are completely flooded, Eggsy uses his bare fist to punch through a large two-way mirror to escape. The fact is that any glass (or plexiglas) thick enough to contain hundreds (or thousands) of tons of water without bursting would be as impenetrable as concrete to Eggsy's bare fist. He would need a chisel-tipped jackhammer to penetrate such a mirror.
Factual error: When the barracks are completely flooded, several of the trainees shove shower hoses down the toilets for an "unlimited supply of air" (Merlin describes it as "simple physics"). However, that old fireman's trick is a technique for surviving dense smoke, which is not under fluid pressure. In the barracks flooded 9 feet deep with water, the pressure would be so enormous that it would easily gush straight down the toilets in a powerful torrent, eliminating any "unlimited air supply."
Continuity mistake: In the scene when Galahad is taking Eggsy below ground at the Kingsman Tailor Shop, they enter the dressing room and the door is open with it still open in the mirror in the next shot. Then when they go below ground it is magically closed.
Other mistake: During the pub fight scene, the guy who got hit with the glass cup (Wound on his forehead), pulls a six shooter out of his pants, but he fires eight bullets at Harry's umbrella.
Character mistake: Galahad claims that classifies Brogues as being 'mutually exclusive' from Oxfords. Broguing is any decorative hole punching on shoes and can be found on any shoe style.
Character mistake: When giving Eggsy a tour, Galahad tells him that Oxfords are any formal shoe with open lacing. This is incorrect as Oxfords have closed lacing.
Visible crew/equipment: When Eggsy is in the car yelling at his mum's boyfriend and the windows start to go up, you can see a man's head next him on the floor.
Continuity mistake: Near the beginning, as Harry gently takes the snowglobe from Eggsy, in the shot of Eggsy the snow is relatively calm, then in the immediate reverse angle as Harry looks at it, the snow is instantly shaken up.
Revealing mistake: In the Kentucky church-massacre scene, Harry briefly scuffles with a man in a light blue sweater. Harry elbows him in the ribs, sticks a grenade in the man's pocket, spins him around and delivers a crushing blow to his neck, then hurls him head-first into a wall. Throughout this sequence, the man is obviously a live human being. However, a fraction of a second before he impacts the wall, he transforms into a life-size rubber dummy before our eyes, in mid-frame, without any obvious camera cut. This demonstrates the complexity of the scene - it is a digitally-edited collage of action sequences almost seamlessly strung together into what appears to be one long traveling shot.
Factual error: Every time the name of the Swedish princess is spoken, it is pronounced "till-dee." This is understandable for the majority of the characters, who speak English as their first language, but in the scene where the Swedish prime minister is interviewed about the disappearance, he also pronounces it "till-dee." He should have used the Swedish pronunciation, "till-deh."
Continuity mistake: When Eggsy and Harry are walking toward the Kingsman tailors just before he gets introduced to the new weapons, there is a long shot of them walking down the street and two silver cars are coming down the street behind them. In the next shot the two silver cars that should still be in the background have disappeared. Also, as they approach the Lexus parked in front of the Tailor's, and there is no car beside them, in a second perspective, there is suddenly a black vehicle parked behind the Lexus.
Other mistake: When the Kingsmen are looking on-screen at the dossier of the princess, the attached photo of her is just a screenshot from the earlier scene where she is at the dining table with Valentine and the Swedish prime minister, just before she is imprisoned. Her photo was not taken at that point, and couldn't have been attached to her private file.
Continuity mistake: Just before the bar brawl, Harry takes the time to lock and bolt every exit and doors so that there would be no possible escape from his wrath...after all is done, and he is done kicking ass he proudly walks away and right through the front door without unlocking it.
Character mistake: Anyone who knows anything about firearms would never handle a firearm with their finger on the trigger unless ready to shoot. In the training scene with the dog Eggsy is running with his finger on the trigger the whole time. Merlin also does it in the end scene at Valentine's secret lair inside the jet.
Continuity mistake: In the end credits scene, when Eggsy appears in the pub, Dean is holding a pint glass in his hand, but in the next shot, the glass is suddenly on the table.
Other mistake: In the final scenes where Eggy goes to have a fight at the pub with his mother's boyfriend they are listening to Slave to Love by Bryan Ferry. The counter says 962 whilst the tape machine is playing (electronic), but the counter does not change.
Answer: It was to test whether the candidates were willing to do whatever it takes to get the job done. If they were willing to shoot the dog they had been caring for, it means they would be willing to make a tough choice in the field to complete a mission.