Continuity mistake: In the fake orgasm scene in the restaurant, when Meg Ryan holds her sandwich up there's one bite out of it, then it flicks on to Billy Crystal and when we see the sandwich again it has two bites out of it, but there was no time for her to have eaten any more. Later it has one bite again.
Continuity mistake: Throughout the movie, Meg Ryan's hair changes styles to depict the different eras. But in a montage of her and Billy Crystal, when it looks like 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.
Continuity mistake: When Harry and Sally are walking to the museum, telling their stories about their sex dreams, Sally does not have a brooch anywhere on her blazer. However, when they are in the museum, she suddenly has a round brooch on the left lapel of her jacket.
Continuity mistake: When Sally is faking her orgasm, the slice of meat on her sandwich changes the way it's folded over the bun between angles.