When Sing fights the gang at the end, before he begins to fight the Beast, he stands on a balcony and throws two bad guys away from him. They land against the wall of the balcony. In the next shot, they crash through the wall and out of the balcony.
When Sing is sitting on the curb after splitting up with his sidekick, his jacket is on the ground beside him. Two guys grab him and take him to the Axe Gang's HQ. He enters wearing the jacket, which had been left behind.
During the first fight with the Beast, the Landlord wraps his hand around the Beast's forearm and turns it sideways. In the next shot, the Landlord makes the same move again, this time on the Beast's hand.
After Brother Sum has covered the mother and child in fuel, the mother's hair drapes down her face and covers her forehead. In the next shot, when Coolie catches the lighter, her hair is combed back out of her face.
When Sing is running away from Landlady, and about to hit a truck, he moves to the left and another truck is there. After he runs underneath the truck and Landlady jumps over it, the other truck disappears.
After the final fight, the camera looks at Beast from the front during a conversation, and his hair is visible. When the camera switches to the rear before he is punched, his the hair is gone, and the man is clearly a stunt double.