When C.C. Baxter gets into Miss Kubelik's elevator to go to the 27th floor, Miss Kubelik pushes a button, presumably 27, which lights up. When they reach their floor, and the elevator doors open, rather than switching off, as an elevator button would normally do when elevator reaches it's destination, the button stays lit, and remains lit through the remainder of the scene.
(Widescreen version only) The shadow of a boom mic is visible in the upper left portion of the screen as C. C. Baxter is rushing to open his apartment door after being alerted to an odor of gas by his landlady.
Fran buys a record album recorded by the small combo that performs in the Chinese restaurant she patronizes but when the record is played in Baxter's apartment, the tunes are lush orchestral arrangements, nothing like the lounge music heard in the café.
When Kirkeby returns to the apartment to look for the galoshes left behind by Sylvia, he first looks carefully behind the chair to the right of the fireplace, then he goes to the left of the fireplace to look behind another chair, then he returns to the right of the fireplace to pick up the galoshes behind the chair where he already looked when he first came in.
During the gin rummy game in Baxter's apartment, he asks Fran how many men there has been in her life and her reply is 3, she also uses her fingers to answer that question, but instead of holding up 3 fingers, she is holding up 4.
When Sheldrake is on the phone requesting Baxter's home phone number, he tells the person he's talking to that Baxter is in ordinary premium accounting, which is not true, as baxter was promoted to an executive by Sheldrake weeks earlier.
After Bud finds out that Ms. Kubelik was with Mr. Sheldrake, he leaves wearing his new hat, leaving his old hat behind. Later that night when he arrives home with the woman from the bar, his old hat is on the hat rack on the wall.
After Baxter gets home for the first time, he cleans up by placing a large waste bucket of empty liquor bottles outside his apartment door. An hour later when he has to vacate the apartment for one of the other men, the waste bucket full of bottles is gone. There was no-one around to carry his bottles away at that time of night.
After Kirkeby leaves, Baxter goes to the kitchen, lights his oven, removes a TV dinner from the refrigerator and puts it in the oven without bothering to set a temperature on the dial. Less than two minutes later, after taking his empty bottles to the hallway and speaking to the doctor, he returns to the kitchen and removes the TV dinner from the oven. Most TV dinners require at least thirty minutes in the oven at a set temperature. Even a modern microwave oven is unlikely to thoroughly cook a TV dinner in less than two minutes.
The layout of Baxter's apartment makes no sense, especially in the context of Dr. Dreyfus's apartment. Dreyfus lives next to Baxter, which means their walls should be adjoining the full length of both flats. But from inside Baxter's living room one can see windows in both his kitchen and bedroom facing directly where the Dreyfus apartment should be (and there would likely be a window in the bathroom between the kitchen and bedroom). Dreyfus's apartment would have to veer immediately off to the extreme right when you enter it and be no more than a couple of inches wide in order to allow the kind of set-up seen in Baxter's apartment - clearly unrealistic, if not downright impossible.
Fran's brother-in-law comes looking for her at her employer's office building after she hasn't been home for 2 days, beginning Christmas Eve. Christmas 1959 was a Friday, so 2 days after she went missing would have been Saturday or Sunday, when nobody would have been in the office.
Baxter finds a piece of spaghetti on the tennis racket he had used earlier in the picture to strain a pot of pasta. While thinking he twirls the noodle around his finger. Pasta left out like that would have stiffened up. And he never would have been able to spin it.
The goof item below may give away important plot points.
At the very end of the film, Baxter and Fran sit down to play a game of Gin. This game is played with 10 cards dealt to each player, but Baxter deals at least 13 cards to both Fran and himself before the picture finally fades out.