Before the kidnapping, when we first see Wng and his wife in the car, as she reclines her seat because she is felling unwell, they are not wearing their seatbelts. In subsequent shots, they are both wearing them.
When Portuguese is being brought in, his hands are cuffed together in front of him. He grabs another set of cuffs from a cop and uses them to fasten the cuffs he's wearing to a copying machine. In the next shot, he is still cuffed to the machine, but his hands are not cuffed together.
Eddie's hair length differs within scenes.
In the beginning, when Eddie is shooting the criminals, he first shoots a guy who falls into a cake shop. After that, when Eddie is hiding behind the car, he fires six more shots without reloading. In the 80s, Hong Kong's CID used .38 revolvers with six bullets.
The model of Wong's car before the kidnapping is newer than the one used when it is being crashed and dumped into the sea.
When Hung stumbles and falls against the front wall of the passenger area of the plane that he and Chan are on there is no bulkhead door that would lead to the cockpit. In fact, there is no door at all; it is just a solid wall.
When Eddie's car rolls, the rig moving the car in the final shot is visible.
During the first shootout, when Chan drops the cartridge cases from his gun, you can clearly see that they are blanks.
No explanation of how they got the victim out of the water near the end.