Revealing mistake: At the end when Li'l Zé (Zé Pequeno) and his crew want Rocket (Buscapé) to take a picture of them in the street, a long-haired boy with a blue shirt is the first to be killed in the final shooting. A couple of seconds before he is shot in the chest, the shape of the blood pack is visible under his shirt.
Deliberate mistake: In many scenes bullets are shown striking things and sparking (this is especially visible on the attack on Knockout Ned's house when a bullet strikes a chair and gives off a huge spark of light). In real life, only special bullets with incendiary materials (meant for starting fires) cause sparks, and this is only when they're fired against metallic objects. It is impossible that a bullet striking a chair would give off a spark.