(at around 1h 12 mins) Harry's hands jump from being neatly interlaced on his lap to sprawled out on the chair arms.
When Mel and Edna are on their way 'to the country' to visit her brother, the camera is following a yellow 1970 Ford LTD across the bridge... when the car turns in at the driveway of the home it's a yellow 1971 LTD.
After Mel (Jack Lemmon) chases the kid (Sylvester Stallone) all over the park and then takes his wallet, he puts it in the right breast pocket of his sport jacket, but when he takes it out to show Edna in the apartment, he removes it from his left breast pocket.
After the robbery, Edna and Mel are sitting in a diner window with foot traffic passing outside. A woman in a white top and red/white checkered paints passes outside traveling right to left. When they leave the diner, the same woman in the same outfit can be noticed again passing them traveling right to left.
When the elevator is broken and Edna (Anne Bancroft) is walking up the 14 flights, the two grocery bags she's carrying change arms. The boxes of baggies and crackers are in the bag in her right arm in the lobby, switch to her left arm when she starts up the stairs, and are back in her right arm mid-climb.
In the intro scene where Jack Lemmon goes to hail a cab, behind him, a female passerby is seen walking towards the camera, wearing an open bright green shirt with mismatched, darker, wide-leg, green pants. A few seconds later, when the P.O.V is from the taxi, filming Jack Lemmon, still in the street, the same passerby in the colorful green outfit is now seen heading the opposite direction, screen-right, revealing that it's an extra, repeatedly passing by the actor to help make up the background crowd.
When Mel starts chasing the Sylvester Stallone character into Central Park, Stallone is bare headed. However, late in the chase, Stallone turns in panic toward Mel and suddenly is wearing a cap.
When Mel is chasing the mugger through Bethesda Terrace in Central Park, that area is completely deserted. In reality there are throngs of people hanging around there on any given day.
By the looks of the pizza the Edisons are eating for dinner, it is most obvious that the interiors were filmed in California. New York pizza most definitely does not look like that.
The apartment building's supposed address, 385 East 88th Street, is actually nearer to First Avenue than to Second.
Edna mentions to her in-laws that she had some jewelry that she could sell, but burglars who stole everything else including their wardrobes surely would have found and stolen her jewelry.
There is no mention of insurance after the burglary, although middle class apartment dwellers would certainly be insured.
When Edna comes home from work with a souffle for dinner, she puts it in the oven but never turns the oven on. A few minutes later when she takes it out of the presumably hot oven, she does not use an oven mitt or pot holder to protect her hand.