When Marcus reads the comic explaining the rise of Big Daddy and Hit Girl, a photo of Red Mist is on the wall, but at that point of the story Red Mist doesn't exist yet.
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.
When the guy exits the bathroom at Rasul's, the cigarette in his mouth disappears and reappears.
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.
When Hit Girl breaks into Frank d'Amico's apartment, the computer screen on Frank's desk disappears when he and Red Mist wait in the office. The screen reappears in following scenes.
The EMTs that responded to Dave's accident put a neck brace on him once he is in the ambulance and right after they remove 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 needs one.
Plates screwed on one's bones do not make them stronger but, on the contrary, more likely to break at unusual places, thus making it more difficult for surgeons to mend new fractures (which is why plates are usually removed a few months after surgery).
When Kick-Ass fires the Mk 153 at the kingpin, it should have exploded on impact and was not a timed fuse. The Mk 153 is designed as a bunker buster. When it hits the target, the round would explode.
Having nerve endings damaged would only bother skin sensitivity, not deep tissue pain sensation, so any blow would still be painful to Dave.
Plates, not left in place for long after surgery, are usually removed once the bones have healed.
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.
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."
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.
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.
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.
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.
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.
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.
After Sal's death, Louis shows Frank D'Amico the photo Louis took on his phone just before he died. It says 7 of 10 above the photo, but it was the last photo he took before he died.
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.
It has been said that Frank has no (confirmed) awareness of Hit Girl's relationship to Big Daddy (her father), so saying "Time for a family reunion" makes no sense; however, while Big Daddy is being burned alive, he shouts "Take cover, child!" Later, Hit Girl is seen on camera. This was shown live on the Internet and it's entirely reasonable to expect that Frank saw these parts of the webcast as he set it up and was shown to be watching before and after.
Actor Stu 'Large' Riley plays "Huge Goon." Nevertheless, though the character is never truly given a name in the film, Frank D'Amico nearly calls him "Stu," only getting out the S before stopping to remark about the bazooka.
Red Mist's gun shoots Hit Girl before it's fully raised and pointing at her.
When the fight takes place in the high rise, the big black guy picks up what he calls a bazooka; however, it is an Mk153 "Bunker Buster."
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.
When Kick-Ass and Hit Girl fly by the Brooklyn Bridge at the end of the film, the World Trade Center can clearly be seen in the background.
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.
When Marcus drops Mindy off at school, reflections of the steadicam and steadicam operator are visible in the car door.
When Kick-Ass meets Red Mist in the car park, the car's logo from the windscreen reflects directly onto Kick-Ass's chest, as if a light was shining at the front of the car. The only light comes from lamp posts high above.
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.
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.
When Dave leaves the house and takes a call from Katie at the Needle Exchange, his cell phone has an 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 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.
When the Mist Mobile first takes off, you can see an HMV store in the background. HMV operates in the UK and Canada (the latter at the time this movie was made, but not anymore) where the film was shot, but it does not operate in the US where the film is set.
Hit Girl knew beforehand the number of goons present at D'Amico's place yet chose to bring with her weapons she had to recharge and not enough ammunition to safely take care of all of the security staff.
Answering an emergency call, it's apparent that the EMTs took unusual (if even nonsensical) care in removing Dave's Kick-Ass wet suit from his body, removing it rather than cutting it from him. It is not clear (because it's not shown) where they did it -- in the street where there was room to maneuver or in the cramped confines of the ambulance, leaving the removal of his mask for last. Dave is later described as being found "naked" in the street (a cover story). It is entirely possibly that his underpants were removed due to the close location of the abdominal stab wound. What a public sight that must/might have been, seeing an injured teen denuded by medical professionals in the street (all but the mask).
When Kick-Ass and Red Mist meet Big Daddy and Hit Girl at the safe house, the boom mic is visible when they stand in the door, at the top of the screen.
When Red Mist shows the Nanny Cam Teddy to Frank, the television reflects the boom mic during the close-up of Mark Strong.
Briefly in the window of a passing taxi as Dave and his friends cross the street to Atomic Comics.
During Kick-Ass' first mission, he is hit by a car just after a car thief stabs him. The scene cuts to Dave in an ambulance, where a paramedic puts a neck brace on him 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.
As Frank is about to shoot Mindy, he says, "Time for a family reunion;" however, Frank has no (confirmed) awareness of Hit Girl's relationship to Big Daddy (her father), nor of her deceased mother.
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.
When Big Daddy enters the room after Marcus breaks in, he clears the handgun, ejecting an "empty" shell casing, not a live round.
It takes two people to properly apply a cervical spine collar -- one firmly holds the head of the patient while the other slips the collar under and around their neck.