When Cliff Robertson goes up to his apartment in the beginning, he goes up to what looks like the ninth floor. When Jane Fonda went into the elevator, she pushed the button for the seventh floor.
When Eileen takes the taxi to her brother's apartment, she gives the address as "120 E. 65th Street," but when she gets there, the address on the building reads "184."
As the four principals drive to Penn Station to drop off Mike Mitchell (Rod Taylor), you can see it pouring rain in the view from the inside of the car, but when they arrive at their destination, the street and surroundings are not wet.
Eileen is wearing white gloves when she gets on the bus before meeting Mike. She still has them on when he boards. He moves to the back of the bus immediately, and in the split second between her reaction and him getting caught on her corsage, her gloves are off and in her hand.
When Mike and Eileen are sitting in the restaurant, they order from a waiter; their conversation is continuous, never showing the waiter come back. Yet when it cuts from the two-shot they're in to a medium shot, there are cups and plates on the table as though their order had been delivered.
About 30 minutes into the film, Eileen is seen standing at the bus stop. In one shot, the back of the sign is visible as she looks up the street to her left for the bus. As the bus arrives, the sign has been turned around to see the front.
In the beginning of the movie when Adam is on the phone talking to Mona there are clocks on the wall he is facing. All the clocks show EST,CST etc indicating the different time zones but, they all show the same time. It looks like they show 4:20.
When Adam is rushing through the airport that is supposedly in the New York area, the colored tile walls are actually those at Los Angeles International Airport (LAX). When he's outside, a reflection of the Theme Building at LAX can be seen in the glass door he opens.
When they are dancing, Eileen says to Russ, "When you came in and saw me and Adam on the couch ..." However, Eileen and Adam (a.k.a. Mike) were standing in the room and were not on the couch when Russ came into the room.