We first see Cary drive a 1955 2 door Lincoln but several scenes later she is driving a 4 door Lincoln. It is possible that Cary, a recent widow, still had both the car she normally drove and her late husbands. This would account for the two different cars.
When Ned mixes a large batch of martinis, the liquid in the shaker is light brown, even though he makes a point of saying he used only two drops of vermouth, which would result in a clear-colored drink in a shaker that big.
Cary reads two quotations from the same page in Walden: "The mass of men lead lives of quiet desperation" and "If a man does not keep pace with his companions..." Those two quotations are from opposite ends of the book, the first and last chapters, respectively.