Continuity mistake: During the first car ride scene Harry is spitting the seeds out the window. One of the shots shown of the car from the outside shows the window still up after he has rolled it down.
Continuity mistake: When they are playing win, lose, or draw, Sally is trying to draw baby talk, and if you keep your eye on the drawing the baby looks different all the time and has eyes, and then doesn't have eyes, etc.
Factual error: When Harry and Sally are driving away from the University of Chicago at the start of their road trip there is a long shot of them driving on Lake Shore Drive, but they are heading south towards the city with the lake on their left. If they were leaving the city from UofC, which is in Hyde Park, they would already be driving away from the city on Lake Shore Drive.
Revealing mistake: In the scene where they have just had sex, Meg goes to get out of the bed and is supposed to be naked, but when she puts on the bathrobe you can see she is wearing some type of slip or nylon panties.
Factual error: When Harry meets Sally for the second time it is on a flight in 1982. But the plane is a second-generation Boeing 737-300, the first of which entered service in December 1984.
Continuity mistake: In the scene where Harry and Sally are having lunch in the diner (the orgasm scene), Harry's napkin is in his lap. You can see it when the camera angle is from the side showing both of them from the knees up. Every time that angle is used, the napkin is in a different place or altogether gone.
Continuity mistake: Coming out of the diner, on the trip from Chicago to New York, when Sally starts opening the door from the inside she does so with her right hand and the purse in her left hand. The next shot is from the outside and now she is pushing the door open using her left hand with the purse hanging from her left shoulder. (00:10:32)
Factual error: When Sally drops Harry off in front of Washington Square Park in Manhattan, the street signs are green with white lettering. However, street signs were yellow with black lettering in Manhattan during the 1970s - yellow street signs weren't replaced with the bright green ones used presently until the mid 1980s.
Continuity mistake: The interior scene on the airplane shows a 707, with the lights mounted on distinctive boxes underneath the overhead bins. The airplane shown landing is a 737, which had the interior lights mounted flush.
Continuity mistake: When Harry and Sally are at a restaurant having lunch (this is when she has the fake orgasm), if you look at Sally's sandwich when they show her from behind, you will see that where she has bit into the sandwich has changed. First the bite is on the right, then the middle, then the left, etc.
Continuity mistake: When Harry and Sally leave the diner on their trip from Chicago to New York, a blue Volkswagon pulls into the parking space next to them. During their dialog outside their car, the blue Volkswagon can be seen behind Sally in the first two shots, but is missing in the last shot of her.
Continuity mistake: When the three women are talking in the open air restaurant, just after Meg broke up with her boyfriend, check the background scenery. It alternates between very cloudy, sunny, and dim and bright, even though the time passed is only seconds.
Continuity mistake: When Harry and Sally are sitting at the table in the diner on their way to New York, Sally's hands change position several times. From the front they will be off the table, in the next shot taken from behind her, they are both on the table. This occurs several times.
Continuity mistake: Also on the trip, there's a conversation scene in the car, in which the camera flips back and forth from Billy to Meg as they chat/argue/flirt. Although the action/dialogue (and therefore the movement of the car) is continuous, the lock button on the driver's side door alternates between "up" and "down" with almost every camera cut.
Continuity mistake: When they stop at the little diner (on the trip to New York) Sally opens her menu twice.
Revealing mistake: During the Lake Shore Drive scene, during the late 1970's, check the model years of the passing cars. There's at least one Ford Taurus which did not debut until the 1984 model year.