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.
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.
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: 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: 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.