The horse drawn carriage near the end of the film passes the same bank twice, going in opposite directions.
When Elliot returns home drunk, prior to opening the window, he knocks over a white table, spilling everything and leaving the table bare. The next shot is of Elliot outside the window, reading a bad review (11 seconds before Paula starts to pull him back inside). The following shot from inside the apartment shows the table with all items returned properly (but not to the same positions). Although it's possible Paula reset the table, it's unlikely that she was able to reach all the items and return them to proper positions so quickly.
When Elliott leaves the apartment to go make the movie at the end of the film, the scarf he is wearing goes from wrapped around his neck, to hanging loose, to wrapped again between shots.
During Paula's audition for Ronnie Burn's production he selects "...the boy in the gray slacks, the girl in the green knee-socks, and the girl in the red scarf." No one on stage is wearing any of these items.
During the spaghetti dinner, Lucy holds a fork wrapped with spaghetti; a second later, in mid-sentence, it is empty.
In the apartment, Elliot and Rhonda claim they are rehearsing Act 1, Scene 4 of Richard III. Neither of their characters appear in that scene. But Elliot also remarks to Paula that he and Rhonda might have other activities in mind, so which scene they're "rehearsing" could be immaterial.
In Elliot's bedroom, he and Rhonda are actually rehearsing act 1/scene 2, not scene 4, of Richard III. But it isn't clear that all they intend to do is rehearse, so which scene they're reading could be unimportant.
When Lucy (Quinn Cummings) pulls the old style flip-top off the can of soda, it breaks leaving a sharp piece sticking out of the opening of the can. Cummings sees it, but continues the scene with barely a flinch. Pretty savvy for a 10-year-old actor.
When Elliott is in the liquor store, the window is simple glass, and there is clearly a gap between the door and the door frame, yet when Paula is right outside being mugged, her screams are completely silent until she opens the door fully.
Based on a previous reference, the movie takes place in the early spring (also note how trees have yet to bud out). Thus, the time period of the film is late March. During a morning conversation between the two lead characters, Paula notes that it's 5:55 a.m. However, the high level of light outside the window would have been impossible at that time of day in the early spring. The scene appears to have been shot in the middle of the day, not so early in the morning.
After the mugging and Elliot confronting the muggers, Paula picks up the last of her groceries from the middle of the road and argues with Elliot. At the top of the screen, a pole can be seen blocking vehicles' entry into the street; in fact a car approaching the intersection from the other side of the cross-street is forced to turn to its left.
Used Best Foods Mayonnaise, instead of Hellmans Mayonnaise. Best foods is West coast, Hellmans is on the East coast.
When the Japanese salesmen exclaim that Paula is making mistakes in her Subaru sales pitch, they use the word "kanchigai," which refers to a misunderstanding or misinterpretation. The more appropriate word for that context would have been "machigai," which denotes an actual error or mistake. This suggests that the Japanese line was written without full awareness of the context in which it was intended to be delivered.
In the last "Richard" rehearsal scene, the director is heard to call out: "Good! Good!" Yet, his lips are clearly seen to be saying: "Yes, Yes!"
The brass, square doorbell button mounted on the door to the apartment sounded with an electric buzzer sound when pushed. This doorbell is actually a mechanical, spring loaded model which sounds a "ding dong" as one pushes the button in and then releases button. The mechanism is on the opposite side of the door and can be seen in later scenes. One typically sees these on office doors, small apartments, mobile homes, etc.
When Paula confronts Elliot for playing his guitar, Dreyfuss' finger movement on the instrument doesn't match the music.
Paula makes sandwiches with Best Foods mayonnaise, but in the eastern U.S., the Best Foods brand is called Hellman's.
When Paula tells Elliot that somebody stole her purse, there is a NYC Police car in the background. Additionally, when Elliot confronts the muggers at the red light, there is another NYC Police car right before the intersection in the left corner.
Elliott cries that the New York Daily News described his interpretation of Richard III as a "splendid Wicked Witch of the North". The witch of the north was Glenda, the Good Witch. On the other hand the New York Daily News is no New York Times either.