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.
Trivia: When they are in the museum and Harry says "For the rest of the day, we are going to talk like this...", the last thing he says is ad libbed. "But I would be proud to partake of your pecan pie." is not in the script. You can see Meg Ryan look off-camera at the director and say "Oh, no" but she continues with him. It's obvious that she wasn't expecting it. [This is confirmed as true in the documentary on the DVD.]
Sally Albright: The first date back is always the toughest, Harry.
Harry Burns: You only had one date. How do you know it's not going to get worse?
Sally Albright: How much worse can it get than finishing dinner, having him reach over, pull a hair out of my head and start flossing with it at the table?
Harry Burns: We're talking dream date compared to my horror.
Sally Albright: At least I got the apartment.
Harry Burns: That's what everyone says. But, really, what's so hard about finding an apartment? What you do is look in the obituary section. You see who died, find out where they lived, and tip the doorman. What they could do to make it easier is combine the two. You know, Mr. Kline died yesterday, leaving behind a wife, two children, and a spacious three bedroom apartment with a wood burning fireplace.