When Lau goes to listen to the "sampler" CD that's been dropped off for him, he presses the button on the player and we see the CD drawer close twice - first in the long-shot, and then in a close-up that starts from a fully-open position on the drawer.
Changes in the time on the clock in the living room when the cop is listening to the CD that's been dropped off for him.
Near the climax, when the lift door opens, the cops handling the pistols don't have their fingers on the trigger - but are just holding the gun.
During the scene in Gallen Garden near the end, the HKSAR flag was flown upside down.
Shawn Yue (Young Chen Wing Yan) is taller than Anthony Chau-Sang Wong (SP Wong Chi Shing), and there is a brief shot of them standing together. Tony Chiu-Wai Leung (Chen Wing Yan) is clearly shorter than Anthony Chau-Sang Wong, so the character has apparently shrunken.