When Harry and Sally enter the diner in 1977, the credit card sticker on the door reads MasterCard. At the time, the company was still known as MasterCharge and the current name was adopted only in 1979.
When Harry and Sally are in the museum, Harry makes a joke about hieroglyphics being part of an ancient comic strip. When he says "...about a character named Sphinxy," his lips don't match the words he can be heard saying.
When Harry and Sally meet for the second time at the airport, Sally mentions that she is twenty-six years old. The subtitle on the next scene, in which Sally discusses her breakup with Joe a few days prior, says "Five Years Later," and Sally mentions that she is thirty-one. In the following scene, Harry and Sally run into each other at a bookstore. Sally tells Marie that the last time she saw Harry was six years ago, not five.
When Harry is dating Emily of Aunt Emily's Desserts, Sally says that 'Harry doesn't even like sweets.' But on New Year's Eve, when Harry is home alone, he is eating Mallowmars and says they're the best cookie ever made.
The first time Harry and Sally stop at a diner (on the way from Chicago to New York), Sally opens the menu, but on the next shot, she opens it again. Her hands are in different positions depending on the camera angle for most of the sequence.
When Harry and Sally are in the diner talking about whether the women have a good time in bed with him, the bite mark in Sally's sandwich keeps moving from being near the side, to the centre, and back to the side.
In 1977, when Harry and Sally enter the diner and also while they eat, Sally's hair is limp and untidy. When Sally is tallying the bill, it is neat freshly styled. When they go outdoors, her hair is untidy again.
When Sally tells Harry that she's watching Casablanca, she says it's on channel 11. A few moments later, when they're both watching the movie in their respective apartments, both TVs are turned to channel 7.
When Harry and Sally drive from the University of Chicago to New York, they should drive on the Lake Shore Drive heading to the south (to the direction of Gary), not to the north (to the direction of downtown). So they should not be on the Lake Shore drive on the north of downtown.
When Harry and Sally are shown leaving Chicago, they are shown driving south on Lake Shore Drive from the north side of Chicago toward downtown even though the University of Chicago is on the south side of the city.
When Sally is faking the orgasm in Katz's Deli, the woman who says "I'll have what she's having" is seated in the self service section and a waiter would not have come over to take here order. The waiter service section is against the wall to Sally's left.
When Jess and Marie are talking to Harry and Sally after they make love, a person walks back and forth in the mirror behind Harry. The person is not a crew member but a restaurant worker as you can also see canned food on shelves.
When Harry, Sally, Jess, and Marie are walking down the street after their dinner, they are all dressed in coats, and Sally is even drawing herself together and raising her shoulders as if from the cold. However, you can see trees with big, lush greenery in the background.
The goof items below may give away important plot points.
At the New Year's Eve party at end of the film when Harry is telling Sally all the things he loves about her, people in the background are still seen dancing even though the music has stopped and you hear the crowd counting down to midnight.
After Harry and Sally have sex, Sally is getting up for some water and is supposed to be naked, however as she stands to put on her robe, you can see her night gown around her legs, which must be tied up around her neck under her hair.