Continuity mistake: At the end of the movie, when Riggs is in the cargo area, a gunman shoots at him, he rolls and starts emptying the clip into him - you see the bullet holes in the gunman. But as Riggs is face to face with the gunman, the holes disappear.
Continuity mistake: While he's on the toilet, Murtaugh is wearing a flak jacket that disappears for one shot while he is getting instructions from the bomb squad leader.
Continuity mistake: In Lethal Weapon 1 Murtaugh turns 50. In Lethal Weapon 2, you can see a copy of Murtaugh's police file in the South African Embassy, when they are discussing repercussions against the police. In the file it says Murtaugh was born in 1947 which would make him 42 in this movie.
Visible crew/equipment: When Riggs first starts shadowing Rudd, he cuts off Rudd's car with his truck. They go to the close up shot of Riggs and when he drives away you can see in the reflection of the trucks window, the PANAV on the hood of the Panavision camera.
Deliberate mistake: The way diplomatic immunity is depicted in the movie is extremely inaccurate. While technically a sufficiently ranking diplomat could perform the acts shown in the movie without being subject to arrest, their home government would be requested to turn the diplomats over for trial, or the diplomat would be declared persona non grata and expelled from the country. A drug smuggling ring and murdering multiple policemen is far, far more serious than actual crimes that diplomats have been expelled (or handed over for trial) for in the real world. Additionally, the consul general and his officers would not be considered diplomats. They would only have limited consular immunity with regards to "official acts" in the course of consular business.
Factual error: All vehicles sold in the U.S. since the 1950s have federally-mandated laminated windshields. When the surfboard crashes through the windshield of the truck, the windshield shatters like ordinary unlaminated glass.
Audio problem: In the scene where Leo explains how he launders the drug money and Roger says "You're a cheat", Leo says "Come on, everyone cheats a little, look at the pentagon." But when he says "look at the pentagon", it's dubbed in because the sound of his voice doesn't match his other dialogue, and if you read his lips it doesn't match either.
Continuity mistake: The right headlight of Murtaugh's car, when it is chasing the BMW, falls off when he hits something, but later on it is fine.
Continuity mistake: In the fight scene toward the end of the film between Riggs and the guy who killed his wife and Rika, look carefully at the watch Riggs is wearing - it moves from one arm to the other in close-up as Riggs forces the guy to stab himself.
Factual error: The amount of money that leaves the cargo container when Riggs and Murtaugh use the car as a battering ram is nowhere near what was inside considering there were at least 4 or so pallets in front of the car. The car didn't have anywhere near the momentum to drive 'through' the pallets and leave them inside.
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.
Continuity mistake: Near the end of the car chase you see a shot of the station wagon Riggs and Murtaugh are riding in get held up by a couple of police cars. The hood is popped open. The shot changes to the BMW crashing into the store window and then back to the station wagon. The hood is down.
Continuity mistake: During a chase scene involving Murtaugh's station wagon and a flatbed tow truck, the hotel waiter shoots at Riggs through the truck's rear window. In this series of shots, notice how Riggs is swinging by a chrome handle on the passenger's side - which wasn't there during the gunpoint hijack shot just moments ago.
Continuity mistake: After Murtaugh confirms the Alba Varden is a ship, one of Rudd's henchmen enters the house armed with a knife. Moments later when they're struggling by the hobby room's blueprint table, Murtaugh grabs the nail gun and shoots him in the forehead. Now right here, you can notice the sound of a knife being thrown to the floor as he reaches for his head. But when the henchman is up against the unhinged doors, he's once again holding a knife in his right hand.
Continuity mistake: When Captain Murphy is briefing Riggs and Murtaugh on their Leo Getz assignment, a small red No Smoking sign can be seen facing outwards on the desk. But as Riggs and Murtaugh stand up to leave, notice how the sign has momentarily turned itself around to face the Captain.
Factual error: Nail guns like the one used as a weapon here can not be fired unless the tip is pressed against a hard surface. It will not fire a nail at someone across any space. The first guy is killed by pressing the gun to his forehead, but the second is hit from several inches away (evidenced by the length of each nail still sticking out of the guy instead of them being fully into his body with only the nail heads showing.
Continuity mistake: Near the end, when Riggs is captured by the bad guys they tie him in a straight-jacket and toss him off a pier into the drink, he's wearing a pair of Dingo style boots. Underwater when he dislocates his shoulder and escapes the straight-jacket, he sees Patsy Kendit's corpse, swims over, checks her pulse and swims to the surface. Now he's barefoot. Yet when he attacks and kills the two thugs he's got the boots on again. They can even be heard on the pavement.