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.
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.
Throughout the retirement dinner, Warren Schmidt is wearing a small gold pin on the left lapel of his suit jacket. When he arrives home from the function and takes the phone call from his daughter, the pin is missing.
When Warren calls Ray on a payphone, a pickup truck that has begun leaving a parking lot in the background in the wide shot is nowhere to be seen in the background a split-second later when the shot goes to close-up.
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.
Warren travels to Holdrege, Nebraska, to visit his childhood home, at "12 Locust Avenue," which he discovers has been replaced by a tire store.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.
The goof items below may give away important plot points.
Warren Schmidt's home address is listed as 5042 Farnam Street for most of the film, but at the end, when he gets the letter from the nun, the address on the envelope is listed as 5402 Farnam Street It is not unheard of for mis-addressed letters to be delivered.