Throughout the movie, Meg Ryan's hair changes styles to depict the different eras. But in a montage of her and Billy Crystal, when she is doing yet another of her long winded menu requests to an exasperated waiter, he hair is straight and almost waist length. This is her hair style at the very end of the movie, when she and Billy are telling about their wedding cake with the coconut and rich chocolate sauce.
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 .
Their cross-country drive supposedly took place in 1976 or '77, so how come when they stop at the diner, the credit card decal on the door advertises "VISA." Bank Americard didn't change its name to VISA until 1979.
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.
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.
When they are with their friends arguing over the ugly coffee table, Sally is wearing black trousers. When she goes outside to talk to Billy Crystal, she's wearing blue jeans.
At the beginning of the movie, they are leaving the campus of the University of Chicago to drive to New York. The next shot is of the car driving north on Lake Shore Drive, north of downtown. This route would take them to Evanston, not New York city.
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.
When Billy Crystal and Meg Ryan drive from Chicago to NYC, she drops him off in front of Columbus Arch. In order for her to be parked in the direction and on the side of the street that she is, she would have had to driven the wrong way against oncoming traffic.