When Cathy and Alan are on the roof of the mansion, Cathy is seen wearing high heeled boots. When the boards they are walking on give way underneath them and they are left hanging from a bar over the water, you can see Cathy's boots no longer have heels.
When Cathy enters her house with her dry cleaning and groceries, her mail slot is visible to the left of her front door. When she comes through the door and falls into the mail, the mail slot is clearly visible on the right side (or left, if you are looking at it from inside the house).
When Rebecca Ryan's signature is shown on receipts for the various outfits purchased, the spelling changes back and forth from Rebecca Ryan to Rebeca Ryan.
When Cathy opens the door and trips on her dry cleaning, she falls facing into the house. When she is shown from inside, she is facing the doorway.
When Rebecca is drunkenly conducting the ensemble playing at the embassy party, the musicians are clearly not playing the Strauss waltz that is heard on the sound track.