Deliberate mistake: In the scene where Mindy saves the dressed-up Dave from the street thugs and almost gets caught by the police. The female voice over the police radio remarks that the suspect is "under five feet" (below 152 cm). As she flees, she uncovered her motorcycle which appears to be a Ducati 1199 Panigale. The Panigale has a seat height of 82.5 cm, which means that by proportion, Mindy cannot sit on the bike with both feet flat on the ground (as a comparison, I'm 5'9 / 175 cm and still have to tiptoe a little bit on the Panigale, so even if she's slightly taller she still couldn't). Yet in the end scene where Mindy drops Dave at home after the final fight, she seems to be able to sustain the bike with both feet only slightly tiptoed, although her boots are not that tall. (00:13:20 - 01:34:30)
Audio problem: When Mindy uncovers her motorcycle and starts it for the first time in the film, the engine sound suggests that the motorcycle (which appears to be a Ducati 1199 Panigale) has a 4-cylinder engine (identifiable by the distinct smooth car-like sound with higher pitch). The Ducati Panigale has a 2-cylinder engine, which should be sounding a lot rougher. But in the end scene where Mindy drops Dave at his house with her motorbike, it makes a completely different sound, which is how the 2-cylinder bike sounds in reality. (00:14:10 - 01:34:25)
Continuity mistake: When Dave is shot by Mindy while training, there are two bullet holes in his jacket when he has only been shot once at that point.
Continuity mistake: When Mother Russia tears off the car door and uses it as a shield, she then throws the door at another police officer and there are no bullet holes/dents visible.
Revealing mistake: When Mother Russia blows up the police car with the gas bottle, both policeman are thrown clear with no damage at all, showing that they came from behind the car and not from inside it.
Factual error: When Mindy kicks one of the guys out of the van, he lands on the windshield of the car behind them and it crumbles. Windshields are not made of safety glass, they are made of laminated glass.
Continuity mistake: When Mother Russia destroys the police cars outside Night Bitch's house she goes from car number 5 to the group of bad guys outside the house in an instant.
Continuity mistake: When Kick-Ass first enters Justice Forever headquarters, there is a jar of peanut butter on the shelf. During his close-up, the peanut butter jar has vanished.
Continuity mistake: Dave goes to the hospital to visit Night Bitch. He said "I'm Kick Ass", while removing his glasses with his left hand. In his right hand, he has flowers. When he sits on the bed he is instantly wearing his glasses again.
