Factual error: The paramedics apply the neck collar to Kick Ass after he was hit by the car. The problem is he is already on the spinal board when it is applied. Neck collars are always applied before putting the patient on the spinal board, especially when head or neck injuries are suspected as would be the case in being struck by a car. Even if there were no witnesses to him being hit, paramedics would assume a neck/head injury if no one saw the incident. Also, the paramedic doesn't immobilize his head before applying the collar, and is far too rough in putting it on.
Continuity mistake: During the scene at Frank's Lumber Supplies, Red Mist's black makeup disappears, only to reappear later.
Revealing mistake: Throughout the movie, people getting shot almost always don't have the shot angles line up with how they appear. This is especially obvious in the end scene with Hit Girl shooting the gangsters: she might shoot someone at an angle from the side, but the shot appears to have come from directly behind the person.
Continuity mistake: Kick-ass grabs the hand of the dude in the red jacket and goes in to bash shoulders with him but in the next second he has the hand of the guy in the green jacket and bashes shoulders with him.
Continuity mistake: In the scene where Red Mist and Kick Ass are in the Mist Mobile in order to go to the warehouse, Red Mist send a text to D'Amico's men. We cut to the warehouse, where one of them is making a joke. He gets the text, reads it for like a second, sends the others away and puts the phone to his ear. Later, when Red Mist plays the video feed to D'Amico, the guy barely looks at his phone before putting it to his ear, then sends his men away.
Continuity mistake: During the scene in the bowling Alley, after Mindy says that she'd like a "Bratz Movie Style Make-Over, Sacha," she places her spoon in her ice cream glass and lets go of it. In the next shot, the spoon is back in her hand, and she is shown placing the spoon into the glass again, without having removed it from before.
Continuity mistake: When D'Amico is looking on his iPod at the picture where he sees kick-ass in the apartment (this is when kick-ass first meets hitgirl) in the next shot its a black iPhone, then an iPod again and then a black iPhone. To give more proof Apple iPhones with black casing are plastic therefore only slightly reflective but you can see on 2 shots the back is silver and reflecting light.
Continuity mistake: During the scene in Razul's apartment the placement of the boxing gloves above the punching bag changes several times throughout the scene. As Kick-Ass confronts Razul they are only barely visible from behind the bag, they switch positions several times during the fight and as Hit-Girl leaves through the window at the end of the scene, they are now on the opposite side of the bag to where they started and in full view.
Other mistake: During Hit Girl's dramatic rescue sequence, she shoots the last two guys in the head. The man she shoots last (in the red and yellow balaclava), she aims at his torso yet shoots him cleanly through the head. It's made all the more obvious by the fact that this scene is in slow motion at this point.