Continuity mistake: When the couple exit the first diner they get in the car and start to talk. The stuff on the back seat changes between angles. (00:12:00)
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. (00:45:10)
Continuity mistake: When Harry and Sally are on their way from Chicago to New York, Sally's hair changes. You have to really look for it, but watch the curl in front appear and then disappear.
Continuity mistake: When the movie starts, Harry is kissing a woman while Sally waits in the car. A couple passes behind the car, but they disappear in the close-up. (00:03:20)
Continuity mistake: In the first deli on the way to New York, Sally slides the menu over the table and places it aligned with the border, very close to her. When the angle changes, it is skewed and several inches away and she is bringing it close again. (00:08:00)
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.
Visible crew/equipment: On their way to New York, the couple stops at a Deli. When they open the car's door, film crew are reflected. (00:07:20)
Answer: Sally could be upset for having to drive Amanda's boyfriend, or it could be awkward because they don't know how to say goodbye. There could be a number of reasons why things appear tense between Sally and Amanda, just not explained.
That was a big question mark for me as well when I watched the movie, to the point that I had to re-watch the beginning because I was sure I had missed something; their behaviour doesn't make sense.
Sammo ★