Plot hole: During Riggs' last fight with Adolph, Adolph holds the knife in his own hand as Riggs forces him to stab himself very slowly. At any point, Adolph could simply have let the knife fall or just tilt down before that happened. Nothing Riggs is doing is making such impossible. "Locking" his wrist does not prevent him from letting go of the knife. It only prevents him flexing his wrist. And this guy is a trained killer, fighting for his life. Better to battle for control of the knife than just let it kill you.
Plot hole: The hitman in the tow truck leans into the rear/left corner of the cab as the surfboard approaches. The board hits the glass at the very top, dead center. Hence, it definitely misses him completely, yet the hitman is killed. Windshields don't break into "shards" (which is another mistake made listed here, and as it can't exist, it can't be the cause of death) and the roof is not damaged. He is the driver, and he stops the truck by braking. There is no vehicle collision. It is clear to all viewers that the makers intended the surfboard to be the cause of death - nobody watching the film thought otherwise.
Plot hole: Riggs and Murtagh send the car flying out of the container, pushing the money out as it goes. Ignoring that the car couldn't possibly force the sealed container doors open, even without having the obstacle of pallets loaded with money in the way, the car actually explodes in mid air for no reason. (Director's Cut.) (01:44:35)
Plot hole: In the opening chase scenes, the bad guys are using hand-held radios to communicate, and several shots show that all the police are hearing them. These professional criminals would not be broadcasting their criminal activities on a police frequency, nor are the police and their dispatchers monitoring scanners to pick up the criminals' frequency. So neither side should be hearing the other. (00:01:50)
Answer: This scene is already in the movie (maybe only the Director's Cut). Riggs surfaces just off the pier, picks up a chain, strangles one man with it and breaks the other man's neck. The scene is cut considerably for play on television and in all versions of the movie in the UK prior to the 2010 UK blu-ray release (including the Directors Cut) - although some TV airings since 2000 have used the full uncut version.