When Jerry, Wade, and Stan are standing outside of the Embers, you can see I-394 over their shoulders. An overhead sign shows an HOV (High Occupancy Vehicle, or carpool) lane. Minnesota introduced HOV lanes in 1991.
When Carl pulls up to the parking gate after stealing the license plate, the airplane on the Northwest Airlines hangar in the background is painted in the 'bowling shoe' livery. This livery was not used by Northwest Airlines until 1989.
Jenny's son has a Whitesnake poster on his wall. The film is set in 1987, which was the peak of Whitesnake's popularity, but the color scheme, logo and font on this poster are consistent with that of the Slip of the Tongue era; that album did not come out until two years later, in 1989.
The irate customer says he was paying "Ninteen Five for that car!". The MSRP for a loaded brand new 1987 Cutlass Cierra was $12,357 which the customer would have seen on the window sticker and far below his original quote.
As Jerry and Jean tell Scotty they don't want him playing hockey, Wade calls Jerry to set up the meeting the next day. When Jean answers the phone, the time on the microwave is 10:44. When she hands the phone to Jerry, it's 7:43. When he turns his back, it's 11:05.
In the opening scene where Jerry is towing the Ciera along the snow-covered roads, both cars are understandably covered with road grime, ice and salt. Later, when he pulls into the King of Clubs for the meeting, both cars are perfectly clean.
As Geaer is running away from Marge onto the frozen lake, he is shot in the leg and falls. The boots and soles are still white. He had just been standing in a mess of blood near the wood chipper, and had blood on his heels as he started to run away.
When Jerry drives the car and trailer into the parking lot, the only light is from the street lamps. When the camera moves from Jerry to some guys playing pool, natural light or daylight is coming in through the window.
During the parking garage sequence, the neon clock at Minneapolis City Hall (visible in some shots) shows a very different time from when the money was supposed to be exchanged. According to the same clock, nearly an hour elapses between Carl's departure and Jerry's arrival.
When Marge is talking to Gaear in the squad car, she asks if the person she saw in the cabin was Jean Lundegaard. However, in the previous scene her body had moved outside and was next in line for the woodchipper treatment.
The sheriff's gun (revolver) changes after she shot Peter Stormare's character - she shoots him in the leg with a snubnose revolver (clearly seen in the close up shot - pun intended) than in the next cut we see a 4 inch barrel revolver in her hand.
When the kidnappers first enter Minneapolis, they are driving northbound on 35W, which means they're approaching from the south. Brainerd is north of the Twin Cities, so they should be approaching from the north.
In Brainerd, when Officer Olson interviews Mr. Mohra about the guy "going crazy at the lake," they each refer to lakes that are nowhere near Brainerd. Officer Olson asks if it's White Bear Lake, about 130 miles southeast of Brainerd. Mr. Mohra tells him it was Moose Lake. Minnesota has more than 1 Moose Lake, but the closest one is in adjacent Cass County, which is outside of the Brainerd Police department's jurisdiction. The error is repeated later, when Marge Gunderson says she is on patrol around Moose Lake.
Nightcrawlers would rarely be used as an ice fishing bait and typically are not even available in the middle of winter. (However although they appear to be in a paper bag, it is foil-lined, which is appropriate for this kind of bait.)
The state trooper turns on the flashing light on the police car way before it gets close to the kidnappers' Cierra. It means that he could not have noticed that the Cierra had dealer plates on it at such distance, at night.
Jean Lundegaard dials 911 from a phone in the upstairs bathroom. A few seconds later, the phone is disconnected. While modern 911 systems could automatically identify the address, that was not the case in 1987. The dispatcher might not have been able to take any further action.
When the patrolman approaches the kidnappers' car at night on the highway, he carries the flashlight in his left hand, and his gun and holster are on his left hip, but when he arrives at the car window, he has the flashlight in his right hand, and the gun and holster are on his right hip. This is not a goof because when the patrolman is approaching the car it's being filmed through the car's side mirror.
Marge is visibly pregnant, but in the initial crime scene, she complains of "morning sickness." Morning sickness typically occurs in the first 12-15 weeks of pregnancy, long before any woman has a visible "baby bump." However, morning sickness can last throughout an entire pregnancy.
In Ludegaard's first plan to raise money, he asks Wade for seed money for a $750,000 parking lot project. Wade refuses to give him the money, but offers him a 10% finder's fee. If he took the finder's fee, he would have had $75,000 instead of the $40,000 he would have had from his second plan of splitting the $80,000 ransom with the kidnappers. However, the ransom demand was $1 million; Jerry only told the kidnappers it was $40,000.
When Jerry speaks to Shep about "the two guys," Shep says he only knows Grimsrud and does not know Carl. But when Shep later bursts into Carl's room to choke and whip him, they clearly recognize each other. Shep could have been lying to Jerry.
During the investigation of the roadside shooting, Marge and her police colleague's uniforms display the American flag incorrectly on their right shoulders. The field of stars is on the left side of the flag, when it should correctly be on the right of the flag on the right shoulder so that it is moving "forwards" - the flag can never retreat. NOTE: While this is true of the US military, it is almost universally common for US law enforcement and firefighters to display flags on their uniforms as depicted in this scene.
At the motel in North Dakota where Jerry is hiding out, the knob on the outside door is not an exterior keyed door lock, but rather an interior type bathroom door knob that can be opened with a paper clip. The hotel employee fakes using a key to open it.
When Gaear Grimsrud does a 180 degree burnout to chase the passersby who saw the dead trooper, the Tan Ciera is spinning its rear wheels, as this maneuver cannot be done with a front-wheel drive vehicle. Later, in the snowy airport parking lot, the front wheels of the Ciera can clearly be seen spinning.
When Marge catches Gaear near the end of the film, he is disposing of Carl's body in a wood chipper and there's a leg sticking out of the hopper which Gaear attempts in vain to force through. Wood chippers do not jam that way; the machine would have made mulch out of Carl in a matter of seconds.
The witnesses to the state trooper shooting at night veer off the road for no apparent reason, after Gaear gives chase. The roads were clear and dry. However, the car was moving at very high speed and the driver was presumably panicked - something as simple as looking over his shoulder at the pursuing headlights could've caused him to drift into the snow on the side of the road and lose control of the car.