The seasons change back and forth, between winter and summer. Sometimes the trees are bare, but in the next scene, they're full again. At the beginning of the movie, the trees are bare and it's gray and cold. Yet, two weeks later he leaves on his trip and it's summer all of a sudden.
During a conversation between Schmidt and his daughter, his sandwich changes from a fairly sparse one to a somewhat plumper one. It subsequently returns to its original thin shape.
In the opening scene, while Warren Schmidt is sitting in his office watching the clock, the telephone, and especially the phone cable, repeatedly change positions on the desk between shots.
The vacuum cleaner behind Jeannie during Warren's lunch moves between shots.
When Warren Schmidt leaves Roberta Hertzel's house to go and sleep in his RV, the RV is noticeably tilted to the right due to its being parked on the side of the street. When he is inside, it is perfectly level.
At the wedding, the priest/minister wears the wrong color of vestments: a purple chasuble and blue stole - purple is for Lent and blue is for Advent. The appropriate color for a wedding in terms of church vestments (be it Catholic, Episcopalian or other) is white.
After Warren leaves the Woodmen building for the last time after visiting his replacement Gary Nordin, he passes the service entrance where all his old files are stacked up in boxes waiting to be trashed. Insurance files with so much confidential data would likely have been incinerated or securely disposed of by an outside company specializing in data removal. Still, the dramatic effect is powerful.
When Warren uses a payphone to call Ray, he is asked to press "3" to erase his message and re-record. He actually presses the # key.
When Vicki gets Warren's jacket, her shirt briefly rolls up, exposing her belly.
When Warren is in Colorado, there are Nebraska license plates on cars parked in driveways.
Scenes showing the University of Kansas are obviously not filmed in Kansas. The real Kansas buildings have red roofs with beige brick.
The Tony Roma's restaurant where the rehearsal dinner is held, is (was, it's now closed), in Omaha, Nebraska, not Denver where the event was held.
Warren travels to Holdrege, Nebraska, to visit his childhood home, at "12 Locust Avenue," which he discovers has been replaced by a tire store. This address does not exist, as there is no Locust Avenue in Holdrege.
In the shot where Warren and Helen pull up to the restaurant for Warren's retirement dinner, the sign writes "retirement" as "retirment."
The key Helen picks out on her way to the car is not a car key.