When Sato has Charlie cornered in the parking garage, and is about to behead him, he looks back at Conklin, and draws a weapon. That looks to be a Wakizashi (Japanese side-sword to accompany the Katana). As he rides off to behead Charlie, he is scraping along the ground a much larger sword (Katana).
The orientation of the shotgun after Sugai's bodyguard tosses it to Nick.
When the Yakuza godfathers arrive for the climactic meeting at the vineyard, the sun is shining as they get out of their cars. In the reaction shot of Michael Douglas watching them, it is unmistakably raining.
When Conklin is taken to the tea-house, one of Sugai's men first throws a pump-action shotgun, then a handful of shells at his feet. As Conklin picks up the shells, a total of six can be counted on-screen. However, in the firefight that follows, he fires only three shots before running out of ammo.
When Miku is visiting the bank to obtain the note plate, in every shot from her flat to the bank she is clearly wearing black hosiery. (Sometimes sheer, sometimes opaque.) As she strips to change, she is already wearing (under the teddy) nude/champagne coloured tights.
Though the movie takes place in Osaka, Japan's second city, none of the Japanese characters speak in the Osaka dialect. Rather, they all speak the standardized dialect, which is based on the Tokyo accent. Persons from Osaka would normally speak standard Japanese to somebody from another region, but it would be almost unheard of, in particular in the underworld, amongst people from Osaka.
When the Osaka police raid a mob hideout, the place is filthy, with garbage everywhere and graffiti on the walls. The director presumably was thinking of a drug den in a slum in the United States. However, it is well known that young recruits to the Japanese yakuza keep hangouts spotless, and are beaten severely if they do not.
In several scenes, Japanese characters (and Nick) can be seen opening and shutting taxi doors. In fact, doors of Japanese taxis are always operated by the driver, through buttons near the driver's seat; so this would never happen. Nick, on the other hand, would not be expected to know this, since it is his first time in Japan.
The paper signed on the airplane in Japanese is a real estate agreement. When they meet at the police station they are told they signed an insurance form. This however could be explained by the Japanese police captain's mixing up the English terms "real estate" and "insurance".
In the movie, the detectives frequent a somewhat swanky looking night club. It is somewhat implausible that relatively low paid police officers would be able to afford a bar like that. In addition, the bar is owned by a known mobster (in fact the mobster even shows up). Japanese police officers would never openly go to a mob owned establishment (or one where mobsters hang out), as could result in a public relations scandal.
The motorcycles are clearly small capacity (125, 250 or so) single cylinder trials bikes, probably two-strokes, but they produce the roar of a four-cylinder liter-size street racer. As part of the Yakuza initiation, new members will alter their bike to make as much noise as possible. They are then required to ride up and down a particular stretch of road revving their engine.
Towards the end of the movie, Nick shoots a hole in a bed sheet hanging on a clothes line, but the pre-cut hole is visible before it is shot.
When Conklin shoots the man with his shotgun, the wire is visible as he gets pulled backward through the door.
Obvious stunt double for Conklin during the motorcycle chase near the end of the film.
At the end where the plates of the $100 bill are shown under the shirt, it clearly shows the "1" of "100" next to the portrait of Franklin, but on the real $100 bill it's the last "0" of "100" next to Franklin.
In the nightclub scene when Masahiro joins Nick and Charlie, while they sitting on the coach. There's a short moment when an extra passing behind Nick shows a middle finger right in camera.
During a gunfight, one or more cars explode in flames, but unless a car is fueled by propane or some other volatile fuel, it shouldn't explode since petrol is merely flammable and not volatile.
During the restaurant scene when Conklin is running out to chase the Yakuza, and fires on them with his weapon, he is using a snub-nose .357. If you watch later in the meatpacking district, when he is looking for Sato, he is only using a .38. He didn't have a second gun, they changed the weapon mid-scene.
The 12 gauge rounds thrown after the pump action shotgun before the final showdown have their primers already stricken, hence have been fired.
In the beginning of the movie when Conklin races the other motorcycle, they shift to a higher gear about 15 times with a 5 gear bike. With that many gears on the back end, they would have been going about 300 MPH.
Towards the end of the film, in the section where Nick and Sato are fighting. Sato pushes Nick onto the bike wheel, and then they cut to Sato punching the ground. In the following shot you see Nick grimacing and in the background you see a blonde haired guy wearing a green tracksuit type top quickly duck behind a tree.
Nick vs. Sato mud fight at the end, Sato slams Nick, Sato throws a punch and misses then Nick gets up and behind him there is a brief appearance of someone moving aside by the tree, most likely a crew member of the movie.
At the start of the Nick versus Sato climatic fight scene, Nick fires his shotgun towards linens hanging on a line. The circular squib wiring is visible on the sheet right before the "bullet" tears a hole in it.
During the mud fight at the end, when the bikes crash, the camera is splashed with drops of mud and one drop is seen briefly traveling down the surface of the lens.
In the final fight Michael Douglas gets kicked in the stomach and hits a tree. During that fight, Douglas is obviously wearing padding under his shirt.
Right after the Stakeout scene, the woman is seen walking away. The scene the cuts from a fishing port in Osaka to what is suppose to be the Osaka City Center area. If you look closely at the signs in the city scene, You would be able to tell that the location was switched to Kobe.
Aside from the fact that police shouldn't frequent a mob nightclub they should also know better than to shout out their full names and professional titles there.