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.
After Mort and Amy exit the elevator, the next shot shows them still getting out.
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.
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 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.
Mort eats corn on a cob while wearing braces. Dentists do not allow patients with braces to eat corn that way, only canned corn off the cob.
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.
When the UPS van drives away after delivering the package, camera and crew are reflection in the side of the van.
When Mort and Shooter are talking next to the highway on the path, a cameraman is visible further back on the trail behind Shooter.