Revealing mistake: Look at Harry and Sally carrying the Christmas tree together. Then jump towards the end of the movie when Sally is dragging on her own a tree for the next holidays. Each garland and even the fake snow covers on the sills are the exact same one year later, down to each crease. At least the vendor changed jackets. (00:48:35 - 01:21:50)
Revealing mistake: 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 panties. (01:11:20)
Revealing mistake: During the Lake Shore Drive scene, during the late 1970's, check the model years of the passing cars. There's at least one Ford Taurus which did not debut until the 1984 model year.
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 ★