In the scene where Wendy Hood has a brief encounter in the middle of the crosswalk with Sandy Carver (at the 19:20 mark), the street corner's curb in the background (behind Sandy) is wheelchair accessible. Such ramps did not exist in 1973.
There is a six-wheeled G.I. Joe "armored personnel carrier" among the numerous toys in the Carver's rec-room. This plastic toy vehicle was not introduced by Hasbro until 1983, ten years after this film is set.
Although this is meant to take place in the 1970s, you can see that the globe does not show the Soviet Union or East/West Germany but instead Germany as a whole and the former republics of the Soviet Union.
There is a yellow plastic Comet container visible near the sink of the Hood home. In 1973 these containers would have been a green cardboard tube with metal top/bottom. Plastic Comet containers did not exist until later.
After her very fumbled sexual encounter in the car parked in the driveway, Elena, in windswept heavy rain, returns to the party house to help her husband who has been violently sick in the downstairs toilet. Though she carried an opened umbrella, her ankle length coat, fully visible onscreen, should have been absolutely soaked and there should have been other evidence of the heavy rain elsewhere on the coat. It was evidently completely dry.
At the key party, when Ben stumbles upon his colleague George Clair while pouring himself a drink, George has a wedding ring on his left hand. But in the next shot, when George's left hand is in the foreground, gesticulating, there is no wedding ring to be seen.
During the opening credits the train conductor announces "...this train originating from New York's Grand Central Station is back in service..." As many people and every train conductor knows the correct name is Grand Central Terminal. This stems from the location on the railroad line; Grand Central is at the end of the line, hence it is a terminal not a station.
When the train arrives, it has the Penn Central logo on the front. At the time, Penn Central operated the commuter railroads throughout the Tri-State Area. However, electric multiple units such as the M-2 Cosmopolitan would not have used that logo. The first batch was owned by the Connecticut Department of Transportation, and all of those trains would've had the state seal on the front.
When Ben begins giving Paul the sex talk in the car, his unease causes him to swerve in to the wrong lane (as indicated by the sound of an oncoming car horn and Ben's swerving back into his lane). Yet, the oncoming car never passes them.