When Chao-an and "The Boss" face off, when "The Boss" pulls out his knives for attack, he slashes Chao-an once across the stomach, and once across the back. However, when slashing at his back, a sound effect implies that he misses, when a scar can be seen on his back seconds later.
In the last fight, the dead henchman in red turns his head from one side to the other from shot to shot. This is most prominent when "The Boss" throws his knife in the end (and the subsequent "kickback").
After the prostitute tells Cheng about the drug smuggling, he leaves. She is sitting with her back to the door when Mi's son sneaks in. You hear the knife flying through the air and see her face as it hits her in the chest. If she had her back to the door, how did the knife embed in her her chest?