A week-old strand of spaghetti on Baxter's tennis racquet is still soft and pliable.
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.
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.
Baxter gives Fran coffee to drink. The coffee is freshly made and so it's boiling hot, but she drinks it quickly, as if it had been sitting around for some time.
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.
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.
During the scene in which Fran cries in front of the mantle, you can clearly someone reflected in the TV screen sitting and watching the scene play out.
At the very end as Baxter deals the cards, Fran drops one on the floor but ignores it.
The frozen daiquiri in the cocktail lounge melts, reforms, and melts again.
During the opening pan of the New York skyline with the United Nations Building in the foreground, the shot is actually a "mirror-image" of the actual scene.
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.
(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.
When Miss Olsen picks up the extension after being fired, we hear Sheldrake completing dialing his call to Baxter. A rotary dialer will not work when the extension is off the hook.
The shaving cream on Baxter's faces changes between the bathroom and bedroom.
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.
Fran's hair keeps shifting and changing during the gin rummy game.
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.
Baxter's pajamas are inconsistent when he is first kicked out of his apartment.
In the opening voice-over, Baxter states the date as November 1, 1959 which was a Sunday. Yet the implication in the narration is that it was Wednesday.
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.
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.
When telling the story of his attempted suicide, C. C. Baxter pantomimes loading a revolver, even though he has specified (and we later see) that the gun he used was an automatic.