Movie Goof (continuity error): While Todd, Marty and Dave were sitting and talking at the booth in the comic store, Marty was reading the comic book, then when the camera changed to show Chris arriving the comic was down on the table.
Movie Goof (continuity error): When Marcus reads the comic explaining the rise of Big Daddy and Hit Girl, a photo of Red Mist on the wall, before Red Mist is introduced in the movie.
Movie Goof (incorrectly regarded as a mistake): 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.
Movie Goof (revealing mistake): In close-ups, none of the vehicles have inspection stickers.
Movie Goof (incorrectly regarded as a mistake): 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.
Movie Goof (errors in geography): When David and his parents are at the breakfast table, a carton of Clover-Stornetta milk is on the table. The film is set in New York; that brand is only sold in parts of California.
Movie Goof (revealing mistake): When Red Mist sends the message from the Mist Mobile on his iPhone, the red exclamation mark appears indicating that the message couldn't be sent. However, his message is still received.
Movie Goof (continuity error): It is nighttime and a black sky in the background through the rear windshield when Red Mist shows off his mist makers on his car. The next second pedestrians wave in bright daylight and it is a blueish morning sky through the rear windshield and above the car. As they drive it goes back to a black sky again.
Movie Goof (incorrectly regarded as a mistake): Red Mist operates an iPhone while wearing gloves. Many gloves have static tips on the fingers, allowing use of static-based touch-screen devices, including the iPhone.
Movie Goof (revealing mistake): 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.
Movie Goof (continuity error): About six minutes in, a shot shows a comic book in Marty's hand. In the next shot, the comic book is on the table, and Marty's hand is on his lap.
Movie Goof (continuity error): When Hit-Girl throws two knives at the gangster, they are an inch or two apart. When the gangster falls, the knives are much farther apart.
Movie Goof (errors made by characters, possibly deliberate errors by the filmmakers): When Kick-Ass leaves home to meet with Red Mist for the first time, he leaves his front door wide open. In the previous scene, his father had left for work.
Movie Goof (revealing mistake): When Dave uses his MacBook, the Apple logo isn't lit up, even thought the computer is supposed to be switched on and in use.
Movie Goof (boom mic visible): Briefly in the window of a passing taxi as Dave and his friends cross the street to Atomic Comics.
Movie Goof (audio/visual unsynchronized): When Mindy pulls the mushroomed slug from her body armor and drops it on the cement, it makes the bright, tinkling sound of an empty brass cartridge.
Movie Goof (continuity error): When Damon asks Mindy what she wants for her birthday, she puts the spoon from her sundae into the bowl. In the next shot, it's back in her hand.
Movie Goof (continuity error): When Dave goes to meet Red Mist one more time, he rides his bike toward the sunrise. His shadow is on his left. If he rides toward the sun, his shadow should be directly behind him.
Movie Goof (continuity error): 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.
Movie Goof (continuity error): When Hit-Girl breaks into Frank d'Amico's apartment, the computer screen on Frank's desk disappears when he and Red Mist are waiting in the office. The screen reappears in following scenes.
Movie Goof (boom mic visible): When Kick-Ass and Red Mist meet Big Daddy and Hit-Girl at the safe house, the boom mic is visible when they're standing in the door, at the top of the screen.
Movie Goof (errors in geography): At the end, a view from a Brooklyn rooftop shows the sun rising over Manhattan. The sun rises in the east, so it would never rise over Manhattan from that perspective.
Movie Goof (revealing mistake): 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.
Movie Goof (continuity error): 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.
Movie Goof (continuity error): During the fight outside the diner the hat that Diner Fight Guy #1 wears at the beginning and end of the fight has vanished during some shots in the latter stages of the fight.
Movie Goof (factual errors): When Big Daddy enters the room after Marcus breaks in he clears the handgun ejecting an "empty" shell casing and not a live round.
Movie Goof (incorrectly regarded as a mistake): 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."
Movie Goof (revealing mistake): 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 due to the fire under Big Daddy and then Hit Girl's flashing light but when Hit Girl shoots her light, extinguishes the fire and goes to shoot out the camera it is mysteriously in night vision mode.
Movie Goof (incorrectly regarded as a mistake): 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.
Movie Goof (revealing mistake): When Dave takes a call from Katie as he's leaving the house and she's at the Needle Exchange, he's using a cell phone clearly branded with the 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 it does not operate in the United States.
Movie Goof (continuity error): When the teens are comparing Kick-Ass and Red Mist in the comic store, it is raining outside. When Katie and David go outside, the sky is clear and the streets have no signs of recent rain.
Movie Goof (continuity error): When Red Mist shows his father the video on the bear, the goon barely seems to look at the message before making the call. In the original shot he stares at it for a couple of seconds.
Movie Goof (continuity error): When Dave rubs self-tanning lotion on Katie, they're both in their underwear. Dave has no scars, despite surviving numerous operations involving metal plates and rods.
Movie Goof (continuity error): When in the Macready's hideout early in the movie, a picture drawn by Big Daddy of Red Mist can be seen, but Red Mist doesn't appear for the first time until later.
Movie Goof (continuity error): During the live execution of Kick-Ass and Big Daddy, blood drips onto Kick-Ass's costume after being beat up but when they are pouring kerosene onto the floor under Kick-Ass and Big Daddy, the blood is gone.
Movie Goof (anachronisms): 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 did start reissuing "retro" blue-on-orange plates in April 2010 but they are of a new style with a blue band across the top, not the 1980s style seen in this film.
Movie Goof (continuity error): When Damon shot Mindy in the chest, she initially fell with her left arm above her head, then when the camera angle changes she had both arms down by her side.
Movie Goof (errors made by characters, possibly deliberate errors by the filmmakers): SPOILER: After Dave is hit by the car, just after the car thief stabs him, the scene cuts to Dave in an ambulance. The 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.
Movie Goof (continuity error): SPOILER: When Kick-Ass and Hit-Girl land on a roof before they unmask themselves, Hit-Girl has knuckledusters on both hands when they are in the air. As soon as they land, both knuckledusters are gone.
Movie Goof (plot holes): SPOILER: During the live "Unmasking of Kick-Ass" webcast, kerosene is splashed all over the floor beneath Big Daddy and Kick-Ass's chairs. But when the kerosene under Big Daddy's chair is lit, the fire remains localized and doesn't spread.