What bugs are there in Chinatown Detective 2?

The biggest bug is the fingerprint of chalk. At the end of the movie, Qin Feng and Yi Song said, I have chalk here. Let's see if it was left by your left hand or your right hand. But when Yi Song talked about "pig food", he was helpless. He drank a glass of water and handed the chalk from his right hand to his left hand before drinking, so this chalk can't be evidence at all.