Revealing mistake: When Oliver breaks into the building and draws his bow, there is a laser pointed at the tip of his arrow. He is told that if he moves a muscle and removes the tip of the arrow out of the of the laser that he will die. But shortly afterwards, just before Oliver looks over at the TV, you can see the arrow tip move out of the way of the laser and see the laser reach the other side of the wall for a split second.
Revealing mistake: When Oliver is threatening Mathis' lawyer, he shoots him in the shoulder. When Oliver grabs the arrow and pushes it through the shoulder, there is a significant difference between the angles. Where the arrow is coming out, it is practically horizontal, but in the chest, it is at a thirty degree angle.
Revealing mistake: After Fires shoots Wilson, he shoots a few more times with the camera on him. Here you can see that the muzzle flash is pretty obviously CGI. Look for things like that fact that it's very small, there's no light shed on the car beside Fires or on himself and there is no smoke.