In the Kick-Ass unmasking shootout scene, Hit-Girl's point of view is too high. She is a relatively small child, but the night-vision shots, supposedly from her point-of-view, show the height of the shooter to be roughly that of the full-grown victims.
Most of the New York license plates are the blue-on-white style in use since 2001, but some are blue-on-orange, which was discontinued in 1986. New York started reissuing "retro" blue-on-orange plates in April 2010, but they are a new style, with a blue band across the top, not the 1980s style seen in this film.
During Kick-Ass' first mission, he is hit by the car just after the car thief stabs him. The scene cuts to Dave in an ambulance, where a paramedic puts a neck brace on Dave after he is on the stretcher. Patients with suspected neck injuries are supposed to be fitted with a neck brace before they are loaded onto a stretcher.
At the warehouse, a goon stares at a text message on his phone for a few seconds, then makes a call. When Red Mist shows his father the video from the bear, the goon barely seems to look at the message before making the call.
When Frank takes Stu's gun to kill the Kick-Ass impersonator, he pulls it from a holster on Stu's right shoulder, meaning that Stu is a left-handed shooter. Later, in the fight in Frank's apartment, we see Stu with his handgun as a right-handed shooter.
After Hit Girl in schoolgirl uniform shoots her way into the lobby of Damico's building, she pulls her roller suitcase and sets it near the elevator perpendicular to the wall. Then Hit Girl is shown at a different angle, and the suitcase is now at an angle and almost parallel to the wall.
In the torture scene when Hit-Girl rescues Big Daddy and Kick-Ass. Before the rescue there is blood on Kick-Ass's mouth and mask. After the rescue, his mouth and mask are clean. In the car journey that follows, there is much more blood around his mouth.
When Kick-Ass meets Red Mist in the car park, the car's logo from the windscreen is reflected directly onto Kick-Ass's chest, as if a light was shining at the front of the car. The only light is coming from lamp posts high above.
When Dave takes a call from Katie at the Needle Exchange as he is leaving the house, his cell phone has a Orange telecommunications company logo, indicating that the phone has been SIM-locked and sold through the company. Orange operates in many parts of the world, notably the UK where the film was largely shot, but not in the United States.
When Kick-Ass and Hit Girl depart by jet-pack after killing Frank D'Amico at the end of the film they are initially flying down University Avenue in Toronto, before the footage cuts to NYC. Canadian flags and the provincial legislature are visible in the background.
The EMTs that responded to Dave's accident put a neck brace on him once he was in the ambulance and they'd removed his superhero mask. EMTs assess c-spine injuries at the scene; they would never transfer a person to an ambulance without a neck brace if he needed one.
In close-ups, none of the vehicles have New York inspection stickers, which should be in the left-hand corner of the windshield, or registration stickers, which should be next to the inspection sticker.
When Kick Ass and Red Mist enter the burning lumber warehouse, they pull open the front doors, which clearly have glass in them. When they exit the same building, they jump through the front doors, which now have no glass in them at all. The fire hasn't reached the doors yet, so the glass should still be there.
In the hallway, Hit Girl dramatically reloads both pistols in slow motion. She takes three shots total before the pistols run out of ammo. Then she throws the guns away and takes one from the next criminal. Hit Girl loaded the magazines herself earlier in the film; she may have ran out of bullets.
Kick-Ass orders a plain green wet suit online, but it arrives with yellow trim. Like other clothing sold online, the picture shows one example of the style. On the ordering page, Green/Yellow Trim is indicated under "Select Style."
When Red Mist and Kick-Ass pull up to Frank's Lumber Yard, the Mustang has scissor doors. When Kick-Ass gets in the car to go meet Hit Girl and Big Daddy, the car doors are normal. When they arrive at the meeting spot, the car has scissor doors again. In the DVD commentary, the director explained that the car had doors that opened either way.
When Frank D'Amico is interrupted by a telephone call, he is training on a Jong (wooden dummy), which is used primarily in Chinese martial arts. He's also wearing a karate Gi, which is used in Japanese martial arts. Many martial artists cross-train between arts. Studying karate wouldn't prevent him from using a Jong for striking.
When Damon shoots Mindy to prepare her to take a bullet while wearing a bulletproof vest, it causes a small bullet hole in the jacket she wears over the vest. It's hard to see because of the jacket's color. Later, the jacket and Mindy's hair hide the three bullet holes.
When Marcus reads the comic explaining the rise of Big Daddy and Hit-Girl, the final pictures of the cartoon do not match those in the comic book Marcus holds a few seconds later. That is because we were seeing a stylized interpretation of the picture "coming to life" into Marcus' imagination.
Red Mist plugs the red and white audio component cables into the bear and the TV, yet he gets both audio and video. The red cable is attached to the bear's yellow (video) port. Since the cables are identical, the red cable on the other end could have been used for the video feed.
When Red Mist shows off the mist makers on his car, the night sky can be seen through the rear windshield. Seconds later, pedestrians wave in bright daylight and a bluish morning sky appears through the rear windshield and above the car. As they drive, the sky turns black again.
The goof items below may give away important plot points.
When Kick-Ass comes out of the bathroom, Hit Girl points the machine gun straight at him. Right after Kick-Ass says, "you can't take that stuff to my house", Hit Girl raises the gun to point it at Kick-Ass.
During the live execution of Kick-Ass and Big Daddy, blood drips onto Kick-Ass's costume after he is beat up. When the thugs pour kerosene onto the floor under Kick-Ass and Big Daddy, the blood is gone.
The video camera streaming The Unmasking of Kick-Ass in color goes black after Hit Girl takes out the lights. We assume the audience sees the ensuing gunfight due to the fire under Big Daddy and then Hit Girl's flashing light. When Hit Girl shoots her light, extinguishes the fire, and goes to shoot out the camera, the broadcast is in night vision mode.
Later in the film, it's hinted and then shown that the narrator has not actually been Kick-Ass in the first person. But earlier the statements made couldn't have been anyone else. Referring to "I" so and so.
When Frank reads the newspaper story about the murder of the Kick-Ass impersonator, the second and third columns show the same article copied twice. However, this copies a common gimmick in comic panels.
After Frank D'Amico kills who he thinks is kick-ass, he later reads in the newspaper that a kick-ass impersonator has been murdered, even though the identity of kick-ass is unknown. How would anyone know that this is just an impersonation rather than the real kick-ass?
During the live "Unmasking of Kick-Ass" webcast, kerosene is splashed all over the floor beneath Kick-Ass and Big Daddy's chairs. When the kerosene under Big Daddy's chair is lit, the fire doesn't spread.