After Mort gets out of his car to confront his wife and her lover at the motel, he closes the car door. It is both heard and can be seen in the rear view mirror. When he returns to leave a moment later, the car door is open.
When everyone is sitting in the insurance adjuster's conference room, Mr. Rainey takes a drink of water from a glass that has obvious condensation on the outside. In subsequent shots, the condensation vanishes, reappears, then vanishes again.
When Mort's wife starts to leave for his house, she puts her purse on her left shoulder and has the divorce papers in her right. When she is at the door, the papers are in the left hand and the purse on the right.
When Mort goes to Ken Karsch's office, Ken hits the button on his clock. The button is on his (Ken's) left. Mort reaches forward and pushes the right button to stop the clock. Later, Ken reaches forward and pushes the right button to start the clock again (this button can't be pushed down; it's already down). After this, the buttons alternate being up and down throughout the scene.
In the scene of Mort at the hotel driving away in his car, he looks in the left hand mirror which has the words "Objects in this mirror are closer than they appear." In actual fact, that label is only on the right hand mirror in cars, and appears to have been applied to the left hand mirror for the movie using a cheap sticker label.
The goof items below may give away important plot points.
When Mort is steering the truck down the path going into the quarry, Greenleaf slumps forward onto the steering wheel, which causes the horn to sound. As Mort continues to steer on the bumpy road, Greenleaf moves forward and backwards, obviously coming off the steering wheel, yet the horn continues to blow.
After Shooter calls Mort and tells him where to go, we see Mort approaching the van, coming close to the driver's seat window (along with a quick glance of the backseat side window) being clean. However, after seeing Greenleaf and Karsch dead, we see a quick cut to the blood-dirtied door windows, a cut to Mort, then a cut to clean door windows.
When Mort's wife tries to escape, she starts her Saab and then struggles to get the manual gear in reverse. This is illogical, as the Saab has its ignition at the gear shifter, incorporating a lock for the shifter. The ignition will always be turned off with reverse engaged, to be able to withdraw the key in these cars.
Mort Rainey and his alter ego John Shooter live in a world of psychosis and hallucination, not bound to reality. Continuity is not required. Many of the scenes from his perspective are filled with deliberate continuity errors, some of which serve as clues and foreshadowing.
Mississippi (where Shooter's car is registered) does not require the use of two license plates. However, since Shooter and his car are part of Mort's imagination, they are not required to adhere to real life.