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.
Other mistake: The first time Kick-Ass tries to stop a crime, when he gets stabbed in the belly and run over by a car, the stick he's carrying is too big to have fitted in his backpack or in his clothing. He could not have been carrying it.
Chosen answer: It's a scar. No explanation is given as to how he got it - presumably it was added to the character's look to suggest a violent past.
Tailkinker ★